Breast Cancer Xenograft Model Research Articles

Reduced Tumor Volume and Increased Necrosis of Human Breast Tumor Xenograft in Mice Pretreated by a Cocktail of Three Specific Anti-HER2 scFvs.

We aimed to assess the effects of a cocktail comprising three specific anti- HER2 scFvs on breast tumor formation in a xenograft mouse model and to evaluate quantitative changes in the tumor using stereological analysis. Three specific anti-HER2 phage antibodies were produced from a scFv-library using phage display technology. The cell binding capacities of the antibodies were assessed via FACS analysis. Soluble forms of the antibodies were prepared by infecting HB2151-E. coli cells and purified using a centrifugal ultrafiltration method. The purification process was evaluated by SDSPAGE analysis. Two forms of scFv cocktails were prepared, soluble scFv and phage-scFv cocktail, which contained an equal amount/phage of each of the three antibodies. Inbred female BALB/c mice were pretreated with 5 and 20 mg/kg of the soluble scFv cocktail and 1011 phage-scFv cocktail/ kg. The mice were then injected with 2×106 SKBR-3 human breast cancer cells. Total tumor, inflammatory and non-inflammatory volumes were estimated using the Cavalieri principle after preparing photomicrograph slides. The anti-HER2 scFvs showed significantly higher binding to SKBR-3 cells compared to the isotype control. SDS-PAGE analysis confirmed the high purification of the scFvs. Stereological analysis revealed that the group pretreated with 20 mg/kg of the soluble scFv cocktail exhibited the highest reductions in total tumor volume, non-inflammatory volume, and inflammatory volume, with reductions of 73%, 78%, and 72%, respectively, compared to PBS-pretreated mice (P-value < 0.0001). The volumetric ratio of necrotic tissue to total tumor volume increased by 2.2-fold and 2- fold in the 20 mg/kg of soluble scFv cocktail and phage-scFv cocktail groups, respectively, compared to the PBS-treated mice (P-value < 0.05). Pre-treatment with a 20 mg/kg anti-HER2 scFv cocktail resulted in a significant reduction in tumor volume and increased necrotic area in a human breast cancer xenograft model, indicating the remarkable anti-tumor effect of the cocktail in vivo.

Oral estrogen receptor PROTAC® vepdegestrant (ARV-471) is highly efficacious as monotherapy and in combination with CDK4/6 or PI3K/mTOR pathway inhibitors in preclinical ER+ breast cancer models.

Estrogen Receptor (ER) alpha signaling is a known driver of ER-positive (ER+)/human epidermal growth factor receptor 2 negative (HER2-) breast cancer. Combining endocrine therapy (ET) such as fulvestrant with CDK4/6, mTOR or PI3K inhibitors is now a central strategy for the treatment of ER+ advanced breast cancer. However, suboptimal ER inhibition and resistance resulting from ESR1 mutation dictates that new therapies are needed. A medicinal chemistry campaign identified vepdegestrant (ARV-471), a selective, orally bioavailable, potent small molecule PROteolysis-TArgeting Chimera (PROTAC®) degrader of ER. We used biochemical and intracellular target engagement assays to demonstrate the mechanism of action of vepdegestrant, and ESR1 wild-type and mutant ER+ preclinical breast cancer models to demonstrate ER degradation-mediated tumor growth inhibition. Vepdegestrant induced ≥90% degradation of wild-type (WT) and mutant ER, inhibited ER-dependent breast cancer cell line proliferation in-vitro and achieved significant tumor growth inhibition (TGI) (87-123%) in MCF7 orthotopic xenograft models, better than the ET agent fulvestrant (31-80% TGI). In the hormone-independent ER Y537S patient derived xenograft (PDX) breast cancer model ST941/HI, vepdegestrant achieved tumor regressions and was similarly efficacious in the ST941/HI/PBR palbociclib-resistant model (102% TGI). Vepdegestrant induced robust tumor regressions in combination with each of the CDK4/6 inhibitors palbociclib, abemaciclib, and ribociclib, the mTOR inhibitor everolimus, and the PI3K inhibitors alpelisib and inavolisib. Vepdegestrant achieved greater ER degradation in-vivo compared to fulvestrant, which correlated with improved tumor growth inhibition, suggesting vepdegestrant could be a more effective backbone ET for patients with ER+/HER2- breast cancer.

An Anti-VEGF-B Antibody Reduces Abnormal Tumor Vasculature and Enhances the Effects of Chemotherapy

The vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs) are key regulators of blood vessel formation, including in tumors, where their deregulated function can promote the production of aberrant, leaky blood vessels, supporting tumor development. Here we investigated the VEGFR1 ligand VEGF-B, which we demonstrate to be expressed in tumor cells and in tumor stroma and vasculature across a range of tumor types. We examined the anti-VEGF-B-specific monoclonal antibody 2H10 in preclinical xenograft models of breast and colorectal cancer, in comparison with the anti-VEGF-A antibody bevacizumab. Similar to bevacizumab, 2H10 therapy was associated with changes in tumor blood vessels and intra-tumoral diffusion consistent with normalization of the tumor vasculature. Accordingly, treatment resulted in partial inhibition of tumor growth, and significantly improved the response to chemotherapy. Our studies indicate the importance of VEGF-B in tumor growth, and the potential of specific anti-VEGF-B treatment to inhibit tumor development, alone or in combination with established chemotherapies.

Folic acid-tripeptide-conjugated synthetic biodegradable nanoparticle-loaded with Ormeloxifene potentially inhibited breast cancer xenograft tumor

ObjectivesOrmeloxifene (OLF) is an estrogen receptor modulator used in breast cancer treatment with highly favorable pharmacodynamics and pharmacokinetic properties. Folic acid (FA) receptor overexpresses in breast cancer cells. Hence, the study was intended to evaluate the therapeutic potential of OLF-encapsulated FA-peptide-conjugated nanoparticles (FA-Pep-OLF-NP) for breast cancer treatment. MethodsWe prepared folic acid tripeptide conjugated OLF-encapsulated PLGA nanoparticles. The surface conjugation of folic acid-tripeptide was assessed by FTIR and XPS analysis. Selectivity, cytotoxicity, apoptosis, mitochondrial depolarization, reactive oxygen species, and cell cycle analysis of the experimental nanoparticles were tested on human breast cancer cells, MDA-MB-231 and MCF-7. The therapeutic efficacy of FA-Pep-OLF-NP and unconjugated nanoparticle OLF-NP was compared in the MDA-MB-231-xenograft tumor animal model. ResultsOLF-NP and FA-Pep-OLF-NP exhibited 4.8 % and 4.5 % w/w of drug loading, respectively. The particles were spherical with a smooth surface and uniform size distribution. FA-Pep-OLF-NP revealed higher cytotoxicity, mitochondrial depolarization, and apoptosis potential in MDA-MB-231 cells compared to MCF-7 cells. Tumor uptake and tumor volume reduction by FA-Pep-OLF-NP and OLF-NP were established in MDA-MB-231 tumor-bearing nude mice. ConclusionFA-Pep-OLF-NP showed preferential delivery of OLF to the breast cancer cells and in breast tumors compared to OLF-NP and showed more drug accumulation in breast cancer xenograft models. FA-Pep-OLF-NP delayed the progress of folate receptor overexpressed breast cancer.

Early therapy evaluation of intra-arterial trastuzumab injection in a human breast cancer xenograft model using multiparametric MR imaging.

To investigate the treatment efficacy of intra-arterial (IA) trastuzumab treatment using multiparametric magnetic resonance imaging (MRI) in a human breast cancer xenograft model. Human breast cancer cells (BT474) were stereotaxically injected into the brains of nude mice to obtain a xenograft model. The mice were divided into four groups and subjected to different treatments (IA treatment [IA-T], intravenous treatment [IV-T], IA saline injection [IA-S], and the sham control group). MRI was performed before and at 7 and 14 d after treatment to assess the efficacy of the treatment. The tumor volume, apparent diffusion coefficient (ADC), and dynamic contrast-enhanced (DCE) MRI parameters (Ktrans, Kep, Ve, and Vp) were measured. Tumor volumes in the IA-T group at 14 d after treatment were significantly lower than those in the IV-T group (13.1 mm3 [interquartile range 8.48-16.05] vs. 25.69 mm3 [IQR 20.39-30.29], p = 0.005), control group (IA-S, 33.83 mm3 [IQR 32.00-36.30], p<0.01), and sham control (39.71 mm3 [IQR 26.60-48.26], p <0.001). The ADC value in the IA-T group was higher than that in the control groups (IA-T, 7.62 [IQR 7.23-8.20] vs. IA-S, 6.77 [IQR 6.48-6.87], p = 0.044 and vs. sham control, 6.89 [IQR 4.93-7.48], p = 0.004). Ktrans was significantly decreased following the treatment compared to that in the control groups (p = 0.002 and p<0.001 for vs. IA-S and sham control, respectively). Tumor growth was decreased in the IV-T group compared to that in the sham control group (25.69 mm3 [IQR 20.39-30.29] vs. 39.71 mm3 [IQR 26.60-48.26], p = 0.27); there was no significant change in the MRI parameters. IA treatment with trastuzumab potentially affects the early response to treatment, including decreased tumor growth and decrease of Ktrans, in a preclinical brain tumor model.

Abstract PO2-05-08: ISM5043, a novel, selective KAT6 inhibitor for the treatment of advanced and refractory ER+ breast cancer

Abstract Breast cancer is the most commonly diagnosed cancer and the second leading cause of cancer death among women in the US. Approximately 70% of human breast tumors are estrogen receptor positive (ER+), and endocrine therapy (ET), which inhibits the estrogen signaling pathway, remains the backbone of therapy for ER+ breast cancer. However, ET resistance often develops, and patients eventually experience disease progression. This is a significant clinical challenge and highlights the urgent need for novel therapies that may help overcome the resistance. KAT6A is a histone (lysine) acetyltransferase (HAT) belonging to the MYST family. KAT6A and its paralog KAT6B play important roles in cell cycle progression, neuron stem cell maintenance and hematopoietic development via acetylation of H3K23. Molecular dysregulation of KAT6A, including amplification and fusion, has been reported in many cancers. In breast cancer, KAT6A is amplified as part of the 8p11 amplicon in about 10-15% of the ER+ breast cancer population, where it functions as an epigenetic modulator of ER expression, and is associated with a worse clinical outcome. Therefore, inhibition of KAT6A may be a promising therapy for ER+ breast cancer. Here, we evaluated ISM5043, a novel, small molecule KAT6 inhibitor for anti-tumor effects in ER+/HER2- breast cancer cell lines and animal models. ISM5043 demonstrated potent inhibitory activity against KAT6A (IC50, 8 nM) and KAT6B (IC50, 16 nM) with selectivity over KAT7 (IC50, 344 nM) and other HATs (IC50 &amp;gt;2000 nM). The compound showed robust, dose-dependent, anti-proliferation activity (IC50 &amp;lt; 10 nM) in multiple ER+/HER2- breast cancer cell lines with KAT6A amplification, in accordance with its suppression of H3K23 acetylation. Investigation of the mechanism of action revealed that treatment with ISM5043 resulted in the downregulation of the expression of ERα in these breast cancer cells. In the ZR-75-1 (ER+, HER2-) xenograft model, ISM5043 demonstrated robust and dose-dependent anti-tumor activity, as monotherapy, with Tumor Growth Inhibition (TGI) between 80~110% at doses ranging from 0.3 to 10 mg/kg, QD. ISM5043 (0.3 mg/kg, QD), in combination with tamoxifen (20 mg/kg, QD) exhibited synergistic effect against tumor growth, with a Q value of 1.25 based on Jin’s equation. Notably, ISM5043 displayed a strong anti-tumor effect (TGI of 67.7% and 83.6% at 3 and 10 mg/kg QD, respectively) as monotherapy, in a patient-derived ER+/HER2- breast cancer xenograft model representing disease progression following multiple prior lines of treatment (including palbociclib in combination with letrozole). ISM5043 also showed favorable ADME properties as well as good pharmacokinetic and safety profiles. Taken together, these data support the clinical evaluation of ISM5043 as a targeted therapy in patients with advanced and refractory ER+ breast cancer. Citation Format: Xin Cai, Xin Cheng, Luoheng Qin, Man Zhang, Junwen Qiao, Supriya Bavadekar, Sujata Rao, Feng Ren. ISM5043, a novel, selective KAT6 inhibitor for the treatment of advanced and refractory ER+ breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-05-08.

Abstract PO3-23-10: Continued treatment with CDK4/6 inhibitors slow tumor progression by extending G1-phase duration in drug resistant populations

Abstract CDK4/6 inhibitors (CDK4/6i) have significantly improved the prognosis of patients with metastatic hormone receptor-positive breast cancer. However, the development of drug resistance leads to the discontinuation of CDK4/6i treatment. The potential benefits of maintaining CDK4/6i treatment in the context of resistance and their underlying molecular mechanisms remain elusive. Here we show that despite tumor progression, continued treatment with CDK4/6i notably extends the G1 phase in drug-resistant cells, thereby slowing their growth. Our single-cell data reveal that the duration of the G1 phase in these resistant cells is approximately tripled under continuous CDK4/6i treatment compared to conditions with CDK4/6i withdrawal, while the durations of the S and G2 phases remain comparable. The maintenance of CDK4/6i induces an alternative inactivation of the retinoblastoma (Rb) protein in drug-resistant cells through its post-translational degradation. However, this alternative form of Rb inactivation is ineffective and thus induces slow activation kinetics of E2F transcription factors, significantly delaying G1 progression. The beneficial effect of maintaining CDK4/6i treatment was further validated in breast cancer xenograft models. Furthermore, our findings suggest that given the spectrum of various CDK4/6i, switching between different CDK4/6i types after the emergence of drug resistance may further delay tumor progression. This study provides new mechanistic insights into the maintenance of CDK4/6i treatment despite the development of drug resistance. Citation Format: Jessica Armand, Sungsoo Kim, Hee Won Yang. Continued treatment with CDK4/6 inhibitors slow tumor progression by extending G1-phase duration in drug resistant populations [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-23-10.

Abstract PO3-18-04: MBRC-101: a novel antibody-drug conjugate (ADC) targeting the membrane-associated tyrosine kinase receptor EphA5 in breast cancer

Abstract Objectives: ADCs have emerged as a major advance in contemporary medical oncology. The receptor tyrosine kinase (RTK) EphA5 possesses the classical receptor kinase topology with an extracellular-binding domain, a single-pass transmembrane domain, and a cytoplasmic kinase domain. We have previously shown that EphA5 is highly expressed in non-small cell lung cancer and contributes to DNA-damage repair and radiation therapy resistance. Others have reported EphA5 expression in gastric, ovarian, and pancreatic cancers. Here, we investigated the expression of EphA5 in breast cancer (BC) and assessed EphA5 as a potential cancer-specific target using a novel ADC designated MBRC-101. Methods: MBRC-101 is composed of a humanized anti-EphA5 IgG1 kappa monoclonal antibody conjugated to the small molecule microtubule-disrupting agent monomethyl auristatin E (MMAE) via a protease cleavable valine-citrulline (vc) ThioBridge linker (ThioBridge™-Glu-[Val-Cit-PAB-MMAE] PEG [24u]). MBRC-101 averages a drug-to-antibody ratio (DAR)=4. The specificity profile of the anti-EphA5 monoclonal antibody was characterized using a Membrane Proteome Array (MPA). Receptor-mediated antibody internalization was determined by real-time live-cell analysis; cell killing assays used both live-cell analysis of cell confluence and the CellTiter-Glo® luminescent cell viability assay. Expression of EphA5 in archival human breast tumor tissue sections was evaluated by immunohistochemistry (IHC) using a commercial polyclonal antibody against human EphA5. In vivo efficacy studies used patient-derived xenograft (PDX) models of BC. Results: IHC demonstrated robust and selective EphA5 expression in 87% (20 of 23) of triple negative breast cancer (TNBC) and 88% (23 of 26) of hormone receptor-positive (HR+) patient tumors tested. EphA5 expression was not detected in adjacent, non-malignant breast tissue. Antibody-specificity profile testing showed that the anti-EphA5 antibody bound exclusively to EphA5 and did not cross-react with other members of the Eph receptor/ephrin ligand family. Binding of the anti-EphA5 antibody to EphA5 on the cell surface triggered rapid internalization and processing of the EphA5/antibody complex through the endosomal-lysosomal system. Consistently, MBRC-101 was cytotoxic to EphA5-expressing cells and its activity was concentration-dependent and commensurate to the levels of EphA5 on the cell surface. In PDX murine models of TNBC, once weekly administrations of intravenous (IV) MBRC-101 showed dose-dependent, robust, and reproducible anti-tumor activity in vivo. Partial tumor responses were observed at doses of 2.5 mg/Kg IV and complete and nearly complete tumor responses starting at 5 mg/Kg IV. Doses up to 10 mg/Kg IV were well-tolerated in tumor-bearing mice with no observable weight loss. Pre-clinical safety confirmed MBRC-101 is also well tolerated in rats and in cynomolgus monkeys. Conclusions: These results support EphA5 as a new and promising membrane-associated molecular target for the design and development of ADC-based therapies against hormone-positive breast cancer and TNBC and support the conduct of a Phase 1/1b study of MBRC-101 in patients with breast cancer. Citation Format: Fernanda Staquicini, Fenny Tang, Vanessa de Oliveira, Daniela Staquicini, Yongjian Wu, Kirstin Barnhart, J Kellogg Parsons, Wadih Arap, Isan Chen, Renata Pasqualini. MBRC-101: a novel antibody-drug conjugate (ADC) targeting the membrane-associated tyrosine kinase receptor EphA5 in breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-18-04.

Abstract PS15-02: A Phase Ⅰb Study of D-0502 as Monotherapy for Advanced or Metastatic ER-Positive and HER2-Negative Breast Cancer: Results from the Dose-Expansion Stage

Abstract Background: Endocrine therapy is the mainstay treatment for estrogen receptor (ER)-positive advanced breast cancer (BC). D-0502, an orally bioavailable selective estrogen receptor degrader (SERD), has previously demonstrated antitumor activity in various ER-positive (ER+) and human epidermal growth factor receptor 2 negative (HER2-) breast cancer cell lines and xenograft models. The phase Ⅰ open-label study (NCT03471663) evaluated the safety, tolerability, pharmacokinetics (PK) and efficacy of D-0502 as monotherapy and in combination with palbociclib in female patients with ER+ and HER2- breast cancer. The dose escalation part of D-0502 monotherapy (phase Ⅰa) has been presented before (SABCS 2021, PS11-26 Abstract #1348). Here, we present the results of D-0502 monotherapy from the dose-expansion stage in phase Ⅰb study. Methods: In the single-agent dose-expansion stage of phase Ⅰb study, patients (n=60) received D-0502 (400 mg QD) in 28-day cycles. Eligible patients included females with confirmed ER+, HER2- locally advanced, or metastatic BC; ECOG 0-1; measurable disease (RECIST v1.1); premenopausal or postmenopausal status; adequate hematologic, hepatic and renal functions. The primary objectives are to characterize the safety of D-0502. The secondary objectives are to evaluate the preliminary anti-tumor activity and the PK characteristic. Results: As of April 07, 2023, 60 female patients were enrolled and treated with 400mg single-agent D-0502. Median age was 57 (range: 32-82) years, 55.0% had an ECOG PS of 1. Disease progression resulting in treatment discontinuation occurred in 66.7% (40/60) of patients, and 6.7% (4/60) of patients discontinued treatment because of AEs. Treatment-related adverse events (TRAEs) were reported in 57 pts (95.0%), most of which were grade 1-2. No grade 4/5 TRAE has been reported. The incidence of serious adverse events was 6.7% (4/60). The most common (≥ 15%) TRAEs included vomiting, dizziness, nausea, increased aspartate aminotransferase and alanine aminotransferase, anaemia, decreased appetite, diarrhoea, and malaise. In the 51 efficacy evaluable patients, 8 patients had partial response (PR) and 27 had stable disease (SD); objective response rate (ORR) and disease control rate (DCR) were 15.7% (8/51) and 68.6% (35/51), respectively. The clinical benefit rate (CBR: CR + PR + SD ≥24 weeks) was 47.1% (24/51). The median PFS was 5.6 months (95% Cl: 2.0, 10.0). Conclusion: D-0502 monotherapy showed promising antitumor activity and tolerable toxicity in female patients with ER+ and HER2- locally advanced or metastatic BC. Currently this treatment is being evaluated in a phase Ⅲ study for patients with ER+ and HER2- locally advanced or metastatic BC in China. Citation Format: Jiayu Wang, Tao Sun, Qingyuan Zhang, Yanxia Shi, Xu Wang, Yiding Chen, Quchang Ouyang, Kunyan Li, Manojkumar Bupathi, w. Jeffery Edenfield, Andrea L.M. Silber, Hong Zong, Erika Hamilton, Dejan Juric, Kate Lathrop, Yihong Zhang, Kathryn Stazzone, Zhe Shi, Yaolin Wang, Ling Zhang, Binghe Xu. A Phase Ⅰb Study of D-0502 as Monotherapy for Advanced or Metastatic ER-Positive and HER2-Negative Breast Cancer: Results from the Dose-Expansion Stage [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS15-02.

Abstract PO2-20-04: TACTIVE-U: phase 1b/2 umbrella study of vepdegestrant, a PROteolysis TArgeting Chimera (PROTAC) estrogen receptor (ER) degrader, combined with other anticancer treatments in ER–positive advanced or metastatic breast cancer

Abstract Background: Vepdegestrant (ARV-471), an oral PROTAC ER degrader, was well tolerated and showed evidence of clinical activity in the first-in-human phase 1/2 study in heavily pretreated patients with ER+/human epidermal growth factor receptor 2–negative (HER2-) advanced breast cancer (NCT04072952). The cyclin-dependent kinase (CDK)4/6 inhibitors abemaciclib and ribociclib are approved in combination with an aromatase inhibitor or fulvestrant, or as monotherapy (abemaciclib), for ER+/HER2- advanced or metastatic breast cancer. In preclinical studies, vepdegestrant combined with abemaciclib or ribociclib showed evidence of synergistic interactions in ER+ breast cancer cells and greater tumor growth inhibition in a xenograft breast cancer model compared with fulvestrant in combination with these agents. Inhibitors targeting different CDKs, such as samuraciclib (oral CDK7 inhibitor), are in clinical development for solid tumors. The open-label, phase 1b/2 TACTIVE-U umbrella study will evaluate the safety, efficacy, and pharmacokinetics of vepdegestrant in combination with other anticancer treatments in patients with ER+ advanced or metastatic breast cancer. Vepdegestrant is being evaluated in combination with abemaciclib (sub-study A; NCT05548127), ribociclib (sub-study B; NCT05573555), and samuraciclib (sub-study C). Trial Design: Patients eligible for current sub-studies of TACTIVE-U are aged ≥18 years, have histologically or cytologically confirmed ER+/HER2- advanced or metastatic breast cancer not amenable to surgical resection, and have received prior therapy for advanced or metastatic disease, including any CDK4/6 inhibitor–based regimen in any setting. In each sub-study, patients will receive vepdegestrant orally once daily (QD) continuously in a dose escalation/de-escalation approach. Abemaciclib will be given orally twice daily continuously in sub-study A, ribociclib will be given orally QD for 21 days followed by 7 days off treatment in sub-study B, and samuraciclib will be given orally QD continuously in sub-study C. For all sub-studies, the primary endpoint of the phase 1b portion is dose-limiting toxicities to determine the recommended phase 2 dose of vepdegestrant in combination with the other anticancer treatment. Secondary endpoints of phase 1b are antitumor activity (objective response, clinical benefit rate [CBR], and duration of response [DOR]), progression-free survival (PFS), safety (type, frequency, and severity of adverse events and laboratory abnormalities), and plasma concentrations of study drugs. The phase 2 portion of each sub-study will further evaluate the antitumor activity of the combinations; the primary endpoint is objective response and secondary endpoints include antitumor activity (CBR and DOR), PFS, overall survival, safety, plasma concentration of study drugs, and changes in circulating tumor DNA. Future combination sub-studies will be included in TACTIVE-U. Citation Format: Claudine Isaacs, Katarzyna Jerzak, John Hilton, José Luiz Miranda Guimarães, Rachel Layman, Dongrui Lu, Gary Mo, Anna Maria Calella, Olga Valota, Sibyl Anderson, Cynthia Ma. TACTIVE-U: phase 1b/2 umbrella study of vepdegestrant, a PROteolysis TArgeting Chimera (PROTAC) estrogen receptor (ER) degrader, combined with other anticancer treatments in ER–positive advanced or metastatic breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-20-04.

Abstract PO2-14-10: A selective MCL-1 inhibitor AZD5991 showed tumor regression in a triple-negative inflammatory breast cancer xenograft model

Abstract Background Triple-negative breast cancer (TNBC) accounts for 15-20% of all breast cancer diagnoses but up to 30% of breast cancer-related deaths. Similarly, Inflammatory breast cancer (IBC) accounts for 2-4% of breast cancer diagnosis but causes 8%–10% of all breast cancer-related deaths. The myeloid cell leukemia-1 (MCL-1) is a member of the BCL-2 family of proteins, which is highly amplified in numerous human cancers and associated with cell immortalization, transformation, and chemoresistance. In fact, MCL-1 is amplified in 55% of TNBC after preoperative chemotherapy. Amongst the MCL-1 inhibitors that have been developed, AZD5991 showed high selectivity and affinity for MCL-1. In myeloma and acute myeloid leukemia cells AZD5991 induces apoptosis and showed anti-tumor effects in in vivo models. In the present study, we evaluated the biological role of the MCL-1 inhibitor AZD5991 in IBC and TNBC in vitro and in vivo models. Materials and methods The expression of the MCL-1 gene was analyzed in the World Consortium IBC patient dataset. Next, we screened a panel of TNBC and IBC cell lines for MCL-1 expression by western blot analysis. The effect of AZD5991 on TNBC and IBC cells was analyzed via cell titer-blue proliferation, clonogenic assays and soft agar assay. To further validate these findings, we used siRNA to knockdown MCL-1 and performed cell proliferation, clonogenic and migration assays. To identify signaling pathways involved in MCL-1 Inhibitor’s effects we performed a proteome profiler human apoptosis array in TNBC cells in the presence of AZD5991. The anti-tumor effect of the MCL-1 inhibitor was evaluated using a SUM-149 (TN-IBC) orthotopic xenograft mouse model. The mice were treated with AZD5991 at doses of 15 and 30mg/kg once weekly via tail vein injection for a duration of 4 weeks. Results The mean expression level of MCL-1 was higher in patients with TNBC than in those with non-TNBC. Further, compared to the patients with ER positive, HER-2 negative tumors those with TNBC showed an increased in the mean expression of MCL-1 gene (p value= 0.05). We screened a panel of TNBC and IBC cell lines in which most of the cell lines showed high levels of MCL-1. The cell proliferation assay showed IC50 ranging from 0.27 – 19.6 μM. The SUM149, SUM190, FCIBC02, MDA-IBC3 and MDA-MB-468 cells showed high sensitivity and showed significant decrease in the proliferation rate while BCX010, KPL4, MDA-MB-231, 4T1.2 and E0771 cells as well as the normal breast cells MCF10A exhibited resistance with no significant decrease in proliferation. The clonogenic and soft agar assays with these sensitive and resistant cell lines showed similar results. Further, these findings were validated via genetic knockdown of MCL-1 which resulted in a significant decrease in cell proliferation, colony formation and migration capability of the TNBC cells. To identify signaling pathways involved in MCL-1 Inhibitor’s phenotypic changes, we performed a proteome profiler human apoptosis array which showed a decreased in BCLx expression, and an increase in expression of BAX and phospho-Rad17 proteins in the AZD5991 treated TN-IBC cells. Finally, in a SUM-149 (TN-IBC) orthotopic xenograft mouse model, mice injected with 30mg/kg showed a significant tumor regression compared to the vehicle-treated mice (vehicle vs 30 mg/kg, p value=0.0006). Conclusions and Future directions Our data demonstrates that AZD5991 inhibits tumorigenesis in an inflammatory breast cancer xenograft model. We are currently validating several significant targets to understand the biological mechanisms related to our candidate drug. Our long-term goal is to develop MCL-1 targeted therapy in combination with standard-of-care treatment for inflammatory and triple-negative breast cancers. Citation Format: Mohd Mughees, Moises Tacam, Alex Tan, Michael White, Bisrat Debeb, Emilly Villodre, Xiaoding Hu, Debu Tripathy, Wendy Woodward, Rachel Layman, Geoffrey Bartholomeusz, Chandra Bartholomeusz. A selective MCL-1 inhibitor AZD5991 showed tumor regression in a triple-negative inflammatory breast cancer xenograft model [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-14-10.

Activation of the Anaphase Promoting Complex Restores Impaired Mitotic Progression and Chemosensitivity in Multiple Drug-Resistant Human Breast Cancer.

The development of multiple-drug-resistant (MDR) cancer all too often signals the need for toxic alternative therapy or palliative care. Our recent in vivo and in vitro studies using canine MDR lymphoma cancer cells demonstrate that the Anaphase Promoting Complex (APC) is impaired in MDR cells compared to normal canine control and drug-sensitive cancer cells. Here, we sought to establish whether this phenomena is a generalizable mechanism independent of species, malignancy type, or chemotherapy regime. To test the association of blunted APC activity with MDR cancer behavior, we used matched parental and MDR MCF7 human breast cancer cells, and a patient-derived xenograft (PDX) model of human triple-negative breast cancer. We show that APC activating mechanisms, such as APC subunit 1 (APC1) phosphorylation and CDC27/CDC20 protein associations, are reduced in MCF7 MDR cells when compared to chemo-sensitive matched cell lines. Consistent with impaired APC function in MDR cells, APC substrate proteins failed to be effectively degraded. Similar to our previous observations in canine MDR lymphoma cells, chemical activation of the APC using Mad2 Inhibitor-1 (M2I-1) in MCF7 MDR cells enhanced APC substrate degradation and resensitized MDR cells in vitro to the cytotoxic effects of the alkylating chemotherapeutic agent, doxorubicin (DOX). Using cell cycle arrest/release experiments, we show that mitosis is delayed in MDR cells with elevated substrate levels. When pretreated with M2I-1, MDR cells progress through mitosis at a faster rate that coincides with reduced levels of APC substrates. In our PDX model, mice growing a clinically MDR human triple-negative breast cancer tumor show significantly reduced tumor growth when treated with M2I-1, with evidence of increased DNA damage and apoptosis. Thus, our results strongly support the hypothesis that APC impairment is a driver of aggressive tumor development and that targeting the APC for activation has the potential for meaningful clinical benefits in treating recurrent cases of MDR malignancy.

DB-1310, an ADC comprised of a novel anti-HER3 antibody conjugated to a DNA topoisomerase I inhibitor, is highly effective for the treatment of HER3-positive solid tumors

BackgroundHER3 (ErbB3), a member of the human epidermal growth factor receptor family, is frequently overexpressed in various cancers. Multiple HER3-targeting antibodies and antibody–drug conjugates (ADCs) were developed for the solid tumor treatment, however none of HER3-targeting agent has been approved for tumor therapy yet. We developed DB-1310, a HER3 ADC composed of a novel humanized anti-HER3 monoclonal antibody covalently linked to a proprietary DNA topoisomerase I inhibitor payload (P1021), and evaluate the efficacy and safety of DB-1310 in preclinical models.MethodsThe binding of DB-1310 to Her3 and other HER families were measured by ELISA and SPR. The competition of binding epitope for DB-1310 and patritumab was tested by FACS. The sensitivity of breast, lung, prostate and colon cancer cell lines to DB-1310 was evaluated by in vitro cell killing assay. In vivo growth inhibition study evaluated the sensitivity of DB-1310 to Her3 + breast, lung, colon and prostate cancer xenograft models. The safety profile was also measured in cynomolgus monkey.ResultsDB-1310 binds HER3 via a novel epitope with high affinity and internalization capacity. In vitro, DB-1310 exhibited cytotoxicity in numerous HER3 + breast, lung, prostate and colon cancer cell lines. In vivo studies in HER3 + HCC1569 breast cancer, NCI-H441 lung cancer and Colo205 colon cancer xenograft models showed DB-1310 to have dose-dependent tumoricidal activity. Tumor suppression was also observed in HER3 + non-small cell lung cancer (NSCLC) and prostate cancer patient-derived xenograft (PDX) models. Moreover, DB-1310 showed stronger tumor growth-inhibitory activity than patritumab deruxtecan (HER3-DXd), which is another HER3 ADC in clinical development at the same dose. The tumor-suppressive activity of DB-1310 synergized with that of EGFR tyrosine kinase inhibitor, osimertinib, and exerted efficacy also in osimertinib-resistant PDX model. The preclinical assessment of safety in cynomolgus monkeys further revealed DB-1310 to have a good safety profile with a highest non severely toxic dose (HNSTD) of 45 mg/kg.ConclusionsThese finding demonstrated that DB-1310 exerted potent antitumor activities against HER3 + tumors in in vitro and in vivo models, and showed acceptable safety profiles in nonclinical species. Therefore, DB-1310 may be effective for the clinical treatment of HER3 + solid tumors.

Abstract LB268: Discovery of a potent, selective and brain penetrating PARP1 inhibitor, XZP-7797

Abstract First-generation PARP1/2 inhibitors are approved for the treatment of cancers, such as ovarian, prostate, pancreas, and breast cancers, with maximal activity against tumors harboring BRCA1/2 mutation or homologous recombination deficiency (HRD). However, PARP1/2 inhibitors in clinical use have severe hematological toxicities due to the inhibition of PARP2, which has been shown to play an important role in the survival of hematopoietic progenitor cells. Furthermore, the current data indicate that synthetic lethality with BRCA mutations is primarily caused by PARP1 inhibition and trapping, whereas PARP2 inhibition and trapping are not critical for anticancer activity. Therefore, the development of selective PARP1 inhibitors is expected to reduce the hematological adverse effects associated with PARP2 inhibition while maintaining the required efficacy. XZP-7797 is a potent, selective, and brain-penetrating PARP1 inhibitor with trapping ability. In biochemical assays, XZP-7797 is more than 1000-fold selective for PARP1 over PARP2 and other members of the PARP family, including PARP3, PARP5a, PARP5b, and PARP6. XZP-7797 can significantly inhibit MDA-MB-436 cell proliferation in vitro (IC50=2.4nM). In MDA-MB-436 tumor cells, XZP-7797 inhibits PARylation and increases γH2AX level in a concentration-dependent manner. XZP-7797 demonstrates rapid tumor regression in a dose-dependent manner in the BRCA1 mutant MDA-MB-436 breast cancer xenograft model. In a pancreatic cancer xenograft model, XZP-7797 can also significantly inhibit tumor growth. To evaluate the ability of XZP-7797 to cross the blood-brain barrier, an intracranial xenograft model of breast cancer is used. The results show that XZP-7797 can significantly inhibit the growth of intracranial tumors in mice at a dose of 10 mg/kg, demonstrating the brain-penetrating ability of XZP-7797. Additionally, XZP-7797 has great PK profiles in preclinical species after oral dosing and good ADME properties. In a 14-day toxicology study of XZP-7797 in SD Rats, XZP-7797 has no hematological adverse effects at the exposure that is 10 times higher than the exposure required to achieve superior efficacy. Citation Format: Yeqing Gao, Yuqian Lu, Bo Chen, Benke Jiang, Feng Wang, Genyan Zhang, Xifeng Ma, Dapeng Zhou, Chunmin Shi, Yunling Wang, Zhipeng Liu, Jun Sun, Huimin Zhou, Mei Xu, Jiakui Li, Bin Liu. Discovery of a potent, selective and brain penetrating PARP1 inhibitor, XZP-7797 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB268.

Abstract LB041: Exploiting a multi-center pre-clinical mouse PDX mini trial to identify new clinical opportunities for protein-drug conjugates (PDCs) targeting ROR1

Abstract The oncofetal cell surface receptor tyrosine kinase ROR1, is expressed on a broad range of solid tumors and hematological malignancies but is largely absent from normal adult tissue. Increased ROR1 expression correlates with poor clinical outcome in certain cancer indications, consistent with its functional role in tumorigenesis, increased metastatic potential and drug resistance. The oncofetal expression pattern of ROR1, coupled with its role in cancer pathology and therapy resistance, and its scaffolding function in caveolae, makes it a highly attractive target for a protein-drug conjugate (PDC) approach. A number of ROR1-targeting ADC and PDC agents are in clinical and pre-clinical development. Understanding the molecular determinants that drive the efficacy of these agents, may open-up patient selection strategies to improve clinical outcomes for ROR1-targeted ADC/PDC therapy. We utilized a small-domain protein-drug-conjugate platform to develop a PDC Candidate Drug (ADP-c389) which produced complete and sustained tumor regressions in patient-derived xenograft (PDX) models of triple negative breast cancer (TNBC) and had an excellent therapeutic index in mice and NHPs. To further inform our clinical planning and assess the broader clinical opportunity afforded by this agent, ADP-c389 was evaluated in a multi-center, multi-indication, 30-PDX mouse mini trial covering 5 different solid tumor indications (lung, ovarian, endometrial, esophageal, gastric). Covariates of target expression, site, model, mouse strain and treatment effect were analysed longitudinally and a multi-metric (TGI, AUC, Best Average Response (BAR), mRECIST) effects analysis undertaken to evaluate response. Efficacy was observed in each of the cancer indications tested, with 77% of models showing an anti-tumor response. Furthermore, the results in NSCLC lung and ovarian PDX models indicated that these are compelling additional cancer types in which to position ADP-c389 clinically. Whilst all responders are ROR1 positive, ROR1 expression itself is necessary but not sufficient to predict the degree of response. Linear Mixed Effect Model (LMM) and multi-omics (RNA-Seq and Proteomics) analysis of the resulting responder/non-responder cohorts has enabled us to determine markers of sensitivity to ADP-c389. In summary, ADP-c389 represents a highly differentiated product for the treatment of a variety of solid tumors. The learnings from this mouse mini trial are also being applied to bi-specific ROR1-based PDCs. Citation Format: Aaron N. Cranston, Mark Wappett, Estelle G. McLean, Chiara Saladino, Aidan McCann, Wei-Wei Kung, Jennifer Thom, Paul Trumper, Andy Porter, Caroline Barelle, Tim Harrison, Graham Cotton. Exploiting a multi-center pre-clinical mouse PDX mini trial to identify new clinical opportunities for protein-drug conjugates (PDCs) targeting ROR1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB041.

Abstract 5911: In vivo efficacy of a novel peptide-conjugated drug in patient-derived xenograft models of breast cancer

Abstract Introduction: Antibody and peptide-conjugated drugs are rapidly entering the clinic for the treatment of patients with early and advanced breast cancer. These drugs are designed to achieve more targeted delivery of cytotoxic agents to tumor cells with robust clinical activity. ARB-1-6, a sortilin (SORT1) targeted peptide conjugated to the cytotoxic agent MMAE, has demonstrated pre-clinical activity and tolerability in cell line xenografts of breast cancer. To further characterize its spectrum of antitumor activity and biomarker correlates we evaluated ARB-1-6 in a collection of clinically and genomically annotated breast cancer patient-derived xenografts (PDX). Methods: Breast cancer PDX models were generated and propagated under IRB approved protocols [14-8358]. ARB-1-6 was dosed in tumor-bearing SCID mice by tail vein injection at 3 mg/kg weekly x 4 doses. Tumor volume was measured twice weekly until day 35 or until models reached humane endpoints, after which tumors were collected. Animals were weighed regularly and assessed for treatment-related toxicity. Results: 7 models (5 TN and 2 HR) have completed efficacy assessment to date. 3/7 models were derived from treatment naïve patients while 4/7 were derived from pre-treated patients (Table 1). Complete response (mRECIST) was seen in 5/7 (71.4%) models with stable disease occurring in 2/7 (28.6%). ARB-1-6 was well tolerated with no weight loss observed. Complete tumor regression was seen in a PDX derived from a patient with clinical resistance to a TROP2 antibody drug conjugate (ADC). Antitumor activity was observed in models with low SORT1 RNA expression; SORT1 IHC and other correlatives studies are underway. Conclusion: ARB-1-6 demonstrates substantial preclinical activity in PDX models at doses that are well tolerated in mice. Treatment of additional models is ongoing. Full results and correlative analyses including relationships between antitumor activity and target protein expression will be presented. PDX Model Characteristics and Patient Treatment PDX Subtype Pt. Treatment 1 TN AC-T 2 TN FEC-D, Gem+Carbo 3 TN AC-T 4 TN No 5 TN No 6 HR No 7 HR TC, ET, Cape, Gem+Cis, TROP2-ADC Citation Format: Mitchell J. Elliott, Meghan Mcguire, Jennifer Silvester, Chantal Tobin, Samah El Ghamrasni, Francine Lui, Andrew Zhai, Ozge Yoluk, Aron Broom, Tracy Stone, Glenn Butterfoss, Serban Popa, Tianyu Lu, Chris Ing, David White, David W. Cescon. In vivo efficacy of a novel peptide-conjugated drug in patient-derived xenograft models of breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5911.

Abstract 3134: SDP03923-000-9106, a novel and optimized LIV-1 ADC with superior anti-tumor efficacy in breast cancer xenograft models

Abstract Background: LIV-1, also known as SLC39A6 or ZIP6, is a transmembrane protein belonging to the zinc transporter family. LIV-1 is a promising target for antibody-drug conjugate (ADC) therapy due to its broad expression in tumors and limited normal tissue expression. Ladiratuzumab vedotin, a LIV-1 targeting ADC with payload of MMAE, is now in phase I clinical trial stage and is the only LIV-1 ADC under clinical development worldwide. The preliminary clinical results demonstrated promising efficacy in TNBC (ORR=32%) but no response in HR+/HER2- breast cancer (ORR=0%), and typical MMAE-related adverse events were observed. Herein, we presented a novel LIV-1 directed ADC, SDP03923-000-9106, consisting of a LIV-1 mAb developed in house conjugated to a proprietary topoisomerase I inhibitor via a cleavable linker with an optimized average drug-to-antibody ratio (DAR) of 6. Method: The in vitro binding affinity of SDP03923-000-9106 against LIV-1 protein was measured by Surface Plasmon Resonance (SPR) assays. The expression levels of LIV-1 in different cancer cell lines were detected by flow cytometry and IHC staining. In vitro activity against cancer cell lines was analyzed by CellTiter Glo assays following 10 days of treatment. The in vivo anti-tumor activities were investigated in both CDX and PDX models of breast cancer. Results: SDP03923-000-9106 is a novel anti-LIV-1 ADC comprised of a humanized LIV-1 monoclonal antibody conjugated to the potent topoisomerase I inhibitor (a novel exatecan derivative designed by Hengrui with a better liposolubility and cellular permeability) by a cleavable linker. SDP03923-000-9106 had an optimal drug-to-antibody ratio (DAR) of 6 and showed strong binding affinity to LIV-1 and favorable internalization. SDP03923-000-9106 exhibited robust anti-tumor activity in cell lines across breast cancer, esophageal squamous cancer, gastric cancer, prostate cancer and non-small cell lung cancer. In the LIV-1 highly expressed breast cancer cell line of MCF-7, SDP03923-000-9106 demonstrated a strong inhibitory effect with IC50 of 5.36 nM. Moreover, in a LIV-1 high-expressing patient-derived xenograft model of breast cancer, SDP03923-000-9106 showed pronounced tumor regression at both doses of 3 and 10 mg/kg and showed stronger anti-tumor activity (TGI: 109%) at the same dose of 3 mg/kg versus BMK-MMAE (TGI: 75%; BMK-MMAE is synthesized by Hengrui using the published structure of Ladiratuzumab vedotin). Conclusion: In summary, SDP03923-000-9106 is a novel LIV-1 targeted ADC with a highly permeable payload demonstrating great anti-tumor activity across multiple types of tumors in preclinical studies. The promising preclinical data support advancing SDP03923-000-9106 into clinical testing, and Investigational New Drug (IND) application to NMPA has been submitted. Citation Format: Yanling Gong, Xing Sun, Changyong Yang, Simeng Chen, Wei Zhang, Cheng Liao. SDP03923-000-9106, a novel and optimized LIV-1 ADC with superior anti-tumor efficacy in breast cancer xenograft models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3134.

Abstract 6068: Discovery of BTX-9341, a bifunctional degrader of CDK4 and CDK6 for HR+/HER2- breast cancer

Abstract CDK4/6 inhibitors (CDK4/6i) such as palbociclib, abemaciclib and ribociclib are used to treat HR+/HER2- breast cancer, but patients can develop resistance via many mechanisms, several of which converge on the upregulation of the cyclin D-CDK4/6 signaling node. This has been shown to limit the effectiveness of CDK4/6i in ER+ breast cancer with up to 20% patients exhibiting innate resistance and up to 70% patients developing acquired resistance after 3 years on therapy (Scheidemann, 2021). To address acquired resistance, we sought a degrader approach. We utilized our PRODEGY platform of Cereblon (CRBN) binders to synthesize CRBN mediated CDK4/6 bifunctional degraders and identified BTX-9341 as a development candidate. Breast cancer cell lines treated with BTX-9341 showed up to 85% degradation of CDK4 and CDK6 with DC50s &amp;lt;1nM. CDK4/6 phosphorylates the protein RB which releases the transcription factor E2F, inducing the expression of genes which promote cell cycle progression. We examined RB phosphorylation by in-cell western, E2F target gene expression by qPCR and cell cycle progression by propidium iodide staining followed by flow cytometry. BTX-9341 was potent in all downstream assays, with phospho-RB IC50s &amp;lt;30nM, E2F target gene downregulation and G0/G1 cell cycle arrest at concentrations as low as 10nM. These downstream effects were sustained throughout 72 hours with BTX-9341 treatment but recovered more rapidly with Palbociclib treatment. We used a 2D colony formation assay (CFA) to assess inhibition of proliferation by cell cycle arrest. BTX-9341 potently inhibited cell proliferation with CFA IC50s of 20-50nM while CDK4/6i had CFA IC50s of 50-1000nM. This increased activity was due to CRBN mediated target degradation, as demonstrated by a shift in CFA IC50 values in a CRBN knockout cell line towards the values seen with the inhibitors. In palbociclib-resistant HR+/HER2- cell line models BTX-9341 maintained a low CFA IC50 (&amp;lt;150nM) while CDK4/6i displayed micromolar CFA IC50s. BTX-9341 displays excellent pharmacokinetic properties which allowed for oral dosing in xenograft studies. In several breast cancer xenograft models, BTX-9341 showed dose-dependent tumor growth inhibition, tumor regression at higher dose levels, and was effective with multiple alternate dosing regimens. These results show that BTX-9341 displays excellent single agent activity in vitro and in vivo and that this activity is maintained in CDK4/6i resistant models. This indicates that a degrader approach to targeting this pathway may be more effective than current therapies, and that using this modality in a post CDK4/6i setting may be more effective than switching CDK4/6 inhibitors. Reference: Scheidemann, Erin R, and Ayesha N Shajahan-Haq. “Resistance to CDK4/6 Inhibitors in Estrogen Receptor-Positive Breast Cancer.” International journal of molecular sciences vol. 22,22 12292. 14 Nov. 2021, doi:10.3390/ijms222212292 Citation Format: Hannah Majeski, Akinori Okano, Kirti Chahal, Angela Pasis, Casey Carlson, Arvind Shakya, Qiao Liu, Shenlin Huang, Aparajita Hoskote Chourasia, Leah Fung. Discovery of BTX-9341, a bifunctional degrader of CDK4 and CDK6 for HR+/HER2- breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6068.

Abstract 3121: Therapeutic potential of a HER3 antibody-drug conjugate for the treatment of HER3-expressing cancers

Abstract To address the need for targeted therapy in the treatment of HER3-positive solid tumors, the human HER3 mAb, seribantumab, was evaluated in the context of an antibody-drug conjugate (ADC). As a proof-of-concept, seribantumab was conjugated with a cleavable valine-citrulline linker and monomethyl auristatin E (MMAE) payload via the stochastic cysteine conjugation method to yield HER3-ADC1. A drug-antibody ratio of 4 was selected for preclinical assessment in HER3-expressing models. Methods: HER3-ADC1 was evaluated in vitro and in vivo, with patritumab deruxtecan (patri-DXd) as a comparator. Binding to BT474 breast carcinoma cells (HER3 high; immunohistochemical (IHC) staining intensity 3+) was measured by flow cytometry. Internalization properties were established using seribantumab and patritumab coupled to Fab-AF488 following 4 hours incubation in SK-BR-3 breast adenocarcinoma (HER3 high; IHC 3+) and HCC1569 breast carcinoma (HER3 low; IHC 0-1+) cell lines. In vitro cytotoxicity was evaluated for HER3-ADC1, isotype-MMAE and free MMAE payload as well as patri-DXd, isotype-DXd and free deruxtecan payload in BT474 cells, SK-BR-3 cells and NCI-H446 lung carcinoma (HER3 low; IHC 0-1+) cells. In vivo anti-tumor activity was assessed for HER3-ADC1, isotype-MMAE, patri-DXd and isotype-DXd in patient derived xenograft (PDX) models of pancreatic (HER3 high; IHC 3+) and breast cancer (HER3 low; IHC 0-1+). Results: HER3-ADC1 binding to cancer cells, endocytosis, MMAE release, and inhibition of proliferation were dependent on HER3 expression. In assays investigating antibody internalization, seribantumab displayed similar internalization capacity as patritumab. For both HER3 mAbs, internalization was greater in a HER3 high than a HER3 low cell line. In cytotoxicity assays, HER3-ADC1 and patri-DXd displayed HER3 expression level-dependent cell killing. HER3-ADC1 outperformed patri-DXd in HER3 high cell lines based on in vitro cytotoxicity. HER3-ADC1 and patri-DXd were ineffective against NCI-H466 cells with low HER3 expression. In a PDX model of pancreatic cancer (HER3 high), HER3-ADC1 induced tumor regression, while isotype-MMAE had only a moderate anti-tumor effect. Both patri-DXd and isotype-DXd had moderate anti-tumor effects, indicating a lack of HER3 target dependence. In a PDX model of breast cancer (HER3 low), HER3-ADC1 and isotype-MMAE lacked anti-tumor activity; isotype-DXd had a moderate anti-tumor effect and patri-DXd had a substantial anti-tumor effect. Conclusions: HER3-ADC1 demonstrated target-dependent in vitro cytotoxicity and in vivo anti-tumor activity in a HER3-expressing pancreatic cancer PDX model. Results from in vitro and in vivo studies highlight the promising therapeutic potential of a seribantumab-based ADC for patients with HER3-expressing cancers. Additional results on the optimization and characterization of HER3-ADC1 will be presented. Citation Format: Thomas O'Hare, Jaclyn Cleveland, Valerie M. Jansen, David Dornan. Therapeutic potential of a HER3 antibody-drug conjugate for the treatment of HER3-expressing cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3121.

Abstract 1899: Novel combinations of aplitabart, a DR5 agonist IgM antibody, with ADCs or chemotherapeutic agents lead to robust anti-tumor responses in solid tumor models

Abstract Agonism of death receptor 5 (DR5), a tumor necrosis factor receptor superfamily member, induces the killing of tumor cells via the extrinsic apoptotic pathway upon receptor multimerization. We have engineered a multivalent IgM DR5 agonist antibody, aplitabart (IGM-8444), that induces robust anti-tumor activity in both in vitro and in vivo preclinical models. We also demonstrated potent synergistic in vitro activity of aplitabart with different classes of chemotherapeutic agents including microtubule inhibitors, nucleoside analogs and topoisomerase inhibitors in a range of solid cancer cell lines. Preliminary results from an ongoing phase 1a/ab clinical study show that aplitabart has an encouraging safety profile and promising activity in combination with FOLFIRI ± bevacizumab in heavily pretreated metastatic colorectal cancer patients. To further explore the combination therapies for aplitabart in lung and breast cancers, we evaluated standard of care chemotherapies, such as paclitaxel, carboplatin, and antibody drug conjugates (ADCs) in vitro on a range of human lung and breast cancer cell lines. We observed dose-dependent synergistic cytotoxic effects of aplitabart on tumor cells when combined with these agents. Additionally, these agents increased the expression of DR5 on tumor cells and could explain their synergistic combinatorial activity with aplitabart. Representative cancer cell lines were selected to evaluate these combinations in vivo using xenograft mouse models where robust anti-tumor responses were demonstrated in the xenograft lung and breast cancer models, resulting in enhanced tumor growth inhibition and extended survival. Our findings demonstrate potent in vitro synergistic cytotoxicity and enhanced anti-tumor response in preclinical models in vivo by combining aplitabart with ADCs, or carboplatin and paclitaxel. These results support the combination of DR5 agonist IgM antibodies with agents known to upregulate DR5 expression on solid tumors. Aplitabart is currently under evaluation in a randomized clinical study in combination with FOLFIRI + bevacizumab in comparison with FOLFIRI + bevacizumab in patients with colorectal cancer (NCT04553692). Citation Format: Mélanie Desbois, Susan E. Calhoun, Kevin C. Hart, Carolyn R. Denson, Poonam Yakkundi, Thomas J. Matthew, Maya K. Leabman, Sanela Bilic, Genevive Hernandez, Eric W. Humke, Albert F. Candia, Bruce Keyt, Angus M. Sinclair, Maya F. Kotturi, Beatrice T. Wang. Novel combinations of aplitabart, a DR5 agonist IgM antibody, with ADCs or chemotherapeutic agents lead to robust anti-tumor responses in solid tumor models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1899.