Abstract 2199: BOS172738: A novel highly potent and selective RET kinase inhibitor in Phase 1 clinical development

3094 Background: Selpercatinib, a first-in-class highly selective and potent RET kinase inhibitor, is approved in multiple countries for the treatment of lung and thyroid cancer with RET fusions and medullary thyroid cancer with RET mutations. We provide an efficacy and safety update with more patients (pts) and longer follow-up (data cut-off: 24Sep2021) in RET fusion+ solid tumors with histologies other than lung/thyroid. Methods: The phase 1/2 LIBRETTO-001 trial (NCT03157128) enrolled pts with locally advanced/metastatic RET fusion+ solid tumors. Following dose escalation, pts received the recommended dose of 160 mg orally twice daily. The efficacy analysis set consisted of pts enrolled ≥6 months (mo) prior to the cut-off date. If a pt achieved response, an additional ≥6 mo follow-up from the initial response was required. There was no additional follow-up required for non-responders. Response was assessed per RECIST 1.1. Primary endpoint was objective response rate (ORR) by independent review committee (IRC). Secondary endpoints included ORR by investigator (INV), duration of response (DoR), progression-free survival (PFS), time to response (TTR), and safety. Results: Forty-five pts with 14 unique RET fusion+ tumor types received ≥1 dose of selpercatinib: 12 pancreatic, 10 colon, 4 salivary, 3 unknown primary, 3 sarcoma, 2 each of breast, carcinoma of the skin, xanthogranuloma, and cholangiocarcinoma, and 1 each of lung carcinoid, rectal neuroendocrine, small intestine, ovarian, and pulmonary carcinosarcoma. Median age was 53 years (range 21-85). Forty-one pts received prior systemic therapy (median prior lines: 2, range 0-9); 31% received ≥3 lines. In 41 efficacy-evaluable pts, confirmed ORR by IRC was 44% (18/41, 95% CI: 29-60). Clinical benefit was observed in 63% (26/41) of pts: 2 complete responses (breast, small intestine), 16 partial responses, and stable disease ≥16 weeks in 8 pts by IRC. Responses were observed across a variety of fusion partners. Median TTR was 1.9 mo by IRC. Median DoR was 24.5 mo (95% CI: 9.2-NE) with 50% (9/18) of responses ongoing at a median follow-up of 14.9 mo by IRC. Median PFS by IRC was 13.2 mo (95% CI: 7.4-26.2), with 34.1% alive and progression-free at a median follow-up of 16.4 mo. No new safety signals were identified in this cohort compared to broader safety database. Three grade 5 AEs were observed (unrelated to treatment by INV), and 4 pts discontinued treatment due to AEs (1 deemed related to treatment by INV). Conclusions: Selpercatinib continued to demonstrate durable antitumor activity in pts with RET fusion+ cancers across multiple tumor types. No new safety signals were identified. These results emphasize the importance of comprehensive genomic profiling to identify actionable oncogenic drivers, including RET fusions. The LIBRETTO-001 study continues to enroll pts. Clinical trial information: NCT03157128.

27P Durability of efficacy and safety with selpercatinib in patients (pts) with RET fusion+ non-small cell lung cancer (NSCLC)

746 Background: Selpercatinib, a first-in-class highly selective and potent RET kinase inhibitor with CNS activity, is approved in multiple countries for the treatment of RET-activated cancers. This update includes more patients (pts, n=52) and 16 months (mo) longer follow-up (data cut-off: 13Jan2023) in RET fusion+ solid tumors other than lung/thyroid with a focus on GI histologies. Methods: The phase 1/2 LIBRETTO-001 trial (NCT03157128) enrolled pts with locally advanced/metastatic RET fusion+ solid tumors. 51/52 pts received the recommended dose of 160 mg orally. The efficacy analysis set consisted of pts enrolled ≥6 months prior to the cut-off date. Primary endpoint was objective response rate (ORR) by independent review committee (IRC). Secondary endpoints included ORR by investigator (INV), duration of response (DoR), progression-free survival (PFS), and safety. Results: Of the 52 pts with RET-activated solid tumors other than lung or thyroid, 51.9% were female with a median age of 54.0 years. The majority (31/52) of patients had GI RET fusion+ tumor types, with pancreatic (N=13) and colorectal (N=13) as the most common (Table). 47 pts received prior systemic therapy (median prior lines: 2, range 0-9); 28.8% received ≥3 lines. In 52 efficacy-evaluable pts, confirmed ORR by IRC was 44.2% (23/52, 95% CI: 30.5, 58.7). ORR by IRC for pts with pancreatic tumors was 53.8% and for colorectal tumors was 30.8%. Clinical benefit (complete response + partial response + stable disease ≥16 weeks) was observed in 67.3% (35/52) of pts. Median DoR was 37.2 mo (95% CI: 13.3, NE) for overall pts (Table). No new safety signals were identified compared to previous reports. 4 grade 5 AEs were observed (unrelated to treatment by INV), and 2 pts discontinued due to treatment-related AEs per INV. Conclusions: Selpercatinib continued to demonstrate durable antitumor activity and tolerable safety in pts with RET fusion+ cancers across multiple tumor types and fusion partners, including difficult-to-treat GI cancers with poor results in standard-of-care options, such as pancreatic and colorectal. These results emphasize the importance of comprehensive genomic profiling across all tumor types in order to identify actionable oncogenic drivers, including RET fusions. Clinical trial information: NCT03157128 .[Table: see text]

RET, a receptor tyrosine kinase (RTK) expressed mainly in neural crest-derived tissues, plays a role in cell growth and differentiation and its physiological activation depends upon binding to the GDNF family. Genetic aberrations leading to constitutive RET activation are well-established as oncogenic events. Activating point mutations of RET, for example, are present in ca. 70% of medullary thyroid carcinoma patients including all hereditary cases, while RET gene rearrangements resulting in production of activated RET fusion proteins occur in approximately 10% of sporadic papillary thyroid carcinomas. More recently, recurring RET gene rearrangements have also been found in 1-2 % of lung adenocarcinomas and subsets of other solid tumors including colorectal and salivary gland carcinomas. Thus RET kinase represents an actionable therapeutic target in multiple clinical settings with high medical need. Consequently several small-molecule inhibitors targeting this kinase have been explored in clinical settings. A common feature of most advanced agents is their lack of selectivity and in particular their potent cross-reactivity against VEGFR2, an RTK whose inhibition is associated with serious, dose-limiting cardiovascular toxicity. Indeed, the high homology between the two kinases renders identification of ATP competitive compounds that selectively inhibit RET over VEGFR2 a highly challenging task. Here we describe the preclinical activity of NMS-E668, a potent and selective ATP-competitive RET inhibitor characterized by favorable activity, selectivity and ADME profiles. Biochemically, NMS-E668 has an excellent selectivity profile against a panel of >50 kinases, notably including >10-fold selectivity over VEGFR2. Selectivity of NMS-E668 for RET vs. VEGFR2 was confirmed in NIH-3T3 cells engineered to express activated forms of these kinases. NMS-E668 potently (IC50 circa 50 nM) and selectively inhibited proliferation of RET-dependent tumor cells, including TT medullary carcinoma cells harboring a RET C634W activating point mutation and LC2/ad lung carcinoma cells bearing the oncogenic fusion protein CCDC6-RET. NMS-E668 also potently inhibited IL3-independent growth of Ba/F3 cells expressing KIF5B-RET, the RET rearrangement that is most commonly found in lung adenocarcinomas. Cellular mechanism studies confirmed that NMS-E668 inhibits RET autophosphorylation and downstream signaling at doses consistent with antiproliferative activity. Tested in vivo against KIF5B-RET-driven Ba/F3 tumors, NMS-E668 displayed >90% tumor growth inhibition accompanied by target modulation following oral administration at 10 and 20 mg/kg, with prolonged tumor regressions observed at the higher dose. Thus NMS-E668, a potent and VEGFR2-sparing RET inhibitor is an innovative and highly promising candidate for further development. Citation Format: Elena Ardini, Patrizia Banfi, Nilla Avanzi, Marina Ciomei, Paolo Polucci, Alessandra Cirla, Antonella Ermoli, Ilaria Motto, Elena Casale, Giulia Canevari, Cinzia Cristiani, Sonia Troiani, Federico Riccardi Sirtori, Nadia Amboldi, Dario Ballinari, Francesco Caprera, Eduard Felder, Arturo Galvani, Daniele Donati, Antonella Isacchi, Maria Menichincheri. NMS-E668, a potent and selective RET kinase inhibitor characterized by specificity towards VEGFR2 and high antitumor efficacy against RET-driven models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2082. doi:10.1158/1538-7445.AM2017-2082

Background: Constitutive activation of RET kinase activity following mutation or rearrangement can lead to the development of cancers such as medullary thyroid carcinoma and lung adenocarcinoma. The currently approved therapeutics for these diseases are significantly compromised due to dose-limiting toxicities associated with off-target activity vs KDR (VEGFR2) and lack of potency vs anticipated secondary resistance (e.g., gatekeeper) mutations. Consequently there is considerable interest in the development of highly selective inhibitors targeting diverse RET alterations including the putative resistance mutation, V804M. Methods: We have established a robust screening cascade complemented by structure-enabled drug design and effective medicinal chemistry. Biochemical activity vs RET, KDR, and RETV804M protein was assessed using a HTRF assay. Cellular activity was quantified in BaF3 cells dependent on activity of RET, KDR, or RETV804M for proliferation. Tumor growth inhibition and supporting PK/PD studies were carried out in a number of disease-relevant models including a KIF5B-RET lung cancer patient-derived xenograft (PDX) model, a medullary thyroid carcinoma (MZ-CRC-1) xenograft model, and a lung cancer control (Calu-6) xenograft model. Results: Using this optimized, robust platform, we have identified a number of selective compounds offering a range of interesting biochemical and cellular profiles, targeting either, or both, RET and the gatekeeper mutant, RETV804M. We believe certain examples of these compounds offer the first cell-active RETV804M-selective derivatives. More importantly perhaps, we have also delivered a highly selective preclinical candidate compound demonstrating potency vs both RET fusion and RETV804M. This compound is well tolerated in vivo after oral dosing at up to 80mg/kg bid and, in a KIF5B-RET lung cancer PDX model, demonstrates efficacy at much lower doses: 50% tumor regression at 20mg/kg bid and 92% tumor growth inhibition at 10mg/kg bid. Importantly, this agent shows no efficacy in the (non-RET driven) Calu-6 xenograft model, demonstrating selective inhibition of the RET kinase domain. Conclusions: We believe that the identification of well-tolerated, selective RET inhibitors with potent activity against diverse RET alterations (including the anticipated resistance mutation, V804M) offers a clear therapeutic advantage over the present clinically approved compounds. Our most advanced compound fulfills all of these challenging criteria and has now entered preclinical development. Citation Format: Mandy Watson, Rebecca Newton, Ben Acton, Helen Small, Habiba Begum, Samantha Hitchin, Paul Kelly, Donald Ogilvie, Ian Waddell, Allan Jordan. Delivery of a potent, selective, and efficacious RET inhibitor for the treatment of RET-driven lung adenocarcinoma [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A123.

Introduction: Selpercatinib, a first-in-class highly selective and potent RET kinase inhibitor, is approved in multiple countries for the treatment of RET fusion-positive lung or thyroid cancers. RET fusions are also implicated in the pathogenesis of other cancers. Selpercatinib's efficacy and safety were thus explored in patients (pts) with RET fusion-positive non-lung/non-thyroid cancers in a global, multicenter, registrational trial. Methods: Adults with locally advanced or metastatic RET fusion-positive non-lung/non-thyroid solid tumors enrolled in the phase 1/2 LIBRETTO-001 trial (NCT03157128) were included in this analysis (data cut-off: 19 March 2021). Following dose escalation, pts received the recommended dose of 160 mg orally, twice daily. Pts enrolled long enough to allow 6-month follow-up from their first dose comprised the efficacy-evaluable population. Response was assessed (RECIST 1.1) by investigators. The primary endpoint was objective response rate (ORR). Secondary endpoints included duration of response (DoR), time to response, and safety. Results: Thirty-two pts with RET fusion-positive non-lung/non-thyroid cancers included 12 unique tumor types: 9 pancreatic, 9 colon, 2 each of breast, salivary, sarcoma, and unknown primary, and 1 each of carcinoid, rectal neuroendocrine, small intestine, xanthogranuloma, ovarian, and pulmonary carcinosarcoma. The median age was 48 years (range 22-85). Twenty-nine pts received prior systemic therapy (median prior lines: 2, range 0-9). The ORR was 47% (N=15/32, 95% CI: 29-65). Objective responses were observed in 9 unique cancer types including colon, pancreatic, carcinoid, small intestine, salivary, xanthogranuloma, breast, ovarian, and sarcoma, and 5 additional patients had stable disease lasting ≥ 16 weeks. Median time to response was 1.9 months (range 0.7-7.3). Median DoR was not reached (median follow-up time of 13 months). Responses were ongoing in 73% (11/15) of pts. Safety among this population was consistent with the overall selpercatinib safety database. No patients in this cohort discontinued due to treatment-related AEs. Conclusion: Selpercatinib demonstrated promising antitumor activity in RET fusion-positive non-lung/non-thyroid cancers, including multiple treatment-refractory GI malignancies. Broad-based genomic profiling is essential to identify actionable oncogenic drivers, including RET fusions. The safety and efficacy of selpercatinib will continue to be explored in pts with these cancers in the ongoing LIBRETTO-001 study. Citation Format: Vivek Subbiah, Bhavana Konda, Todd Bauer, Caroline McCoach, Gerald Falchook, Masayuki Takeda, Jyoti Patel, Jared Weiss, Nir Peled, Lyudmila Bazhenova, Victoria Soldatenkova, Pearl French, Nora Drove, Oliver Gautschi, Alexander Drilon. Efficacy and safety of selpercatinib in RET fusion-positive cancers other than lung or thyroid cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr CT011.

2199: BOS172738: A novel highly potent and selective RET kinase inhibitor in Phase 1 clinical development

Background: Activating mutations and oncogenic fusions of the RET receptor tyrosine kinase have been identified in multiple tumor types, including thyroid, lung, breast and colon carcinoma. Furthermore, tyrosine kinase inhibitors (TKIs) with anti-RET activity have produced clinical responses in patients whose tumors harbor RET alterations. However, currently available RET inhibitors were initially developed to target kinases other than RET and are only moderately potent against RET, inhibit multiple kinases other than RET or poorly inhibit secondary resistance mutations (e.g. gatekeeper mutations) common to other TKIs. We have discovered novel, potent and selective RET inhibitors. The resulting compounds exhibit nanomolar potency against wild type RET and select RET mutants, including the KIF5B-RET fusion and V804M gatekeeper mutation, in both enzyme and cellular assays, with minimal activity against highly related kinases. AR025 is representative of this series; the activity of AR025 and related analogs in relevant in vitro and in vivo models will be presented here. Methods: In vitro and in vivo evaluations, including enzyme and cell-based assays, pharmacokinetic (PK)/pharmacodynamics (PD) correlations, drug metabolism characterization, and non-clinical safety evaluation, were conducted using standard methods. Tumor growth inhibition and PD studies were carried out using subcutaneous allografts of NIH-3T3 cells expressing KIF5B-RET in nude mice. Results: AR025 demonstrated nanomolar potency against both wild type and mutant RET proteins in enzyme and cellular assays. AR025 had minimal activity against an enzyme panel of >200 diverse kinases and demonstrated >50x cellular selectivity against VEGFR, with similar selectivity against other related kinases. AR025 possessed low intravenous clearance and high oral exposure in mice, rats and dogs. Finally, a single oral dose of 30mg/kg produced more than 90% inhibition of phospho-RET in NIH-3T3-KIF5B-RET mouse allografts, while twice-per-day continuous dosing resulted in greater than 90% tumor growth inhibition. Notably, AR025 was minimally toxic at doses up to 100mg/kg per day. Conclusions: We have identified a series of potent and selective RET inhibitors with high oral bioavailability and favorable PK properties in animals. One of these, AR025, demonstrated potent inhibition of RET in enzyme and cellular assays, with minimal activity against highly related kinases. In an NIH3T3-KIF5B-RET allograft model, AR025 effectively inhibited phospho-RET and caused dramatic tumor growth inhibition without significant toxicity. The identification of potent and selective RET inhibitors with significant in vivo activity and minimal toxicity may overcome the limitations of currently available inhibitors with anti-RET activity. Citation Format: Barbara J. Brandhuber, Nisha Nanda, Julia Haas, Karyn Bouhana, Lance Williams, Shannon Winski, Michael Burkard, Brian Tuch, Kevin Ebata, Jennifer Low, Francis Sullivan, Lauren Hanson, Tony Morales, Guy Vigers, Jessica Gaffney, Ross D. Wallace, James Blake, Yutong Jiang, S. Michael Rothenberg, Steven Andrews. Identification and characterization of highly potent and selective RET kinase inhibitors for the treatment of RET-driven cancers. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B192.

JICC01.14 Efficacy and Safety of Pralsetinib in Chinese Patients with Advanced RET Fusion+ Non-Small Cell Lung Cancer after Platinum-Based Chemotherapy

Selpercatinib, a first-in-class, highly selective, and potent CNS-active RET kinase inhibitor, is currently approved for the treatment of patients with RET fusion-positive non-small-cell lung cancer (NSCLC). We provide a registrational data set update in more than double (n = 316) of the original reported population (n = 144) and better characterization of long-term efficacy and safety. Patients were enrolled to LIBRETTO-001, a phase I/II, single-arm, open-label study of selpercatinib in patients with RET-altered cancers. An analysis of patients with RET fusion-positive NSCLC, including 69 treatment-naive and 247 with prior platinum-based chemotherapy, was performed. The primary end point was objective response rate (ORR; RECIST v1.1, independent review committee). Secondary end points included duration of response (DoR), progression-free survival (PFS), overall survival, and safety. In treatment-naive patients, the ORR was 84% (95% CI, 73 to 92); 6% achieved complete responses (CRs). The median DoR was 20.2 months (95% CI, 13.0 to could not be evaluated); 40% of responses were ongoing at the data cutoff (median follow-up of 20.3 months). The median PFS was 22.0 months; 35% of patients were alive and progression-free at the data cutoff (median follow-up of 21.9 months). In platinum-based chemotherapy pretreated patients, the ORR was 61% (95% CI, 55 to 67); 7% achieved CRs. The median DoR was 28.6 months (95% CI, 20.4 to could not be evaluated); 49% of responses were ongoing (median follow-up of 21.2 months). The median PFS was 24.9 months; 38% of patients were alive and progression-free (median follow-up of 24.7 months). Of 26 patients with measurable baseline CNS metastasis by the independent review committee, the intracranial ORR was 85% (95% CI, 65 to 96); 27% were CRs. In the full safety population (n = 796), the median treatment duration was 36.1 months. The safety profile of selpercatinib was consistent with previous reports. In a large cohort with extended follow-up, selpercatinib continued to demonstrate durable and robust responses, including intracranial activity, in previously treated and treatment-naive patients with RET fusion-positive NSCLC.

3MO Genomic evolution of non-small cell lung cancer during the establishment and propagation of patient-derived xenograft models

Introduction: CDK7 is a key regulator of transcription and cell cycle progression and has been implicated in multiple tumor types driven by aberrant transcriptional control (e.g. MYC-, ESR1-activation) and/or aberrant cell cycle control (e.g. RB1, CCNE1, CDKN2A alterations). SY-5609 is an oral, potent, and highly selective CDK7 inhibitor that is advancing through IND-enabling studies to support initiation of a planned Phase 1 oncology trial in early 2020. Here we report on the relationship between pharmacokinetics (PK), pharmacodynamics (PD), and tumor growth inhibition (TGI) in xenograft models of tumor types with transcriptional and/or cell cycle aberrations including high grade serous ovarian cancer (HGSOC), small cell lung cancer (SCLC), triple negative breast cancer (TNBC), and estrogen receptor positive breast cancer (ER+BC). Methods: The relationship between SY-5609 PK, PD, and TGI was evaluated in xenograft models of HGSOC (OVCAR3) and TNBC (HCC70) using once daily (QD) or twice daily (BID) dosing via oral gavage. SY-5609 TGI was evaluated as a single agent (SA) QD in patient-derived xenograft (PDX) models of HGSOC (n=3), SCLC (n=4), TNBC (n=4), and in combination with once weekly fulvestrant in ER+BC PDX models selected in vivo for resistance to the CDK4/6 inhibitor palbociclib (n=1) or resistance to both palbociclib and fulvestrant (n=1). Results: SY-5609 plasma exposure was dose proportional and did not accumulate after repeated therapeutic doses. Dose-dependent transcriptional responses in xenograft tissue were observed within 4 hours of SY-5609 dosing and were sustained for 24 hours. TGI was dose-dependent, with tumor regressions observed at doses significantly below the maximum tolerated dose (MTD). Similar TGI was seen when the same daily dose was administered either QD or BID suggesting that the effect was driven by overall exposure or minimum concentration. In HGSOC, SCLC, and TNBC PDX models, SA SY-5609 induced >50% TGI in all models tested (11/11), with 7/11 (64%) demonstrating robust anti-tumor activity (≥90% TGI or regression): 3/3 HGSOC, 2/4 SCLC, and 2/4 TNBC. In a palbociclib-resistant ER+BC PDX model, the combination of SY-5609 and fulvestrant induced significant TGI (89%), with no evident tumor regrowth up to 21 days after dosing cessation, distinguishing the observed effects from SY-5609 SA or fulvestrant SA. In a palbociclib and fulvestrant double-resistant ER+ BC PDX model, SY-5609 SA resulted in 33% TGI and fulvestrant SA had no activity. In contrast, the combination of SY-5609 and fulvestrant demonstrated significant TGI (68%; p<0.0001 vs fulvestrant SA), suggesting re-sensitization to fulvestrant. Conclusions: We have characterized PK, PD, and anti-tumor activity of the CDK7 inhibitor SY-5609 in a series of xenograft models. Exposure in plasma is dose-proportional and does not accumulate at therapeutic dose levels. SY-5609 induces dose-dependent transcriptional responses in tumor xenograft tissue and shows robust TGI, including regressions, in PDX models derived from multiple solid tumor indications. These results highlight the broad potential for SY-5609 across a variety of solid tumor types, including treatment resistant ER+BC, and support the development of SY-5609 in early phase clinical trials. Citation Format: Liv H Johannessen, Shanhu Hu, Nan Ke, Anthony D'Ippolito, Nisha Rajagopal, Jason Marineau, Anneli Savinainen, William Zamboni, Graeme Hodgson. Preclinical evaluation of PK, PD, and antitumor activity of the oral, non-covalent, potent and highly selective CDK7 inhibitor, SY-5609, provides rationale for clinical development in multiple solid tumor indications [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C091. doi:10.1158/1535-7163.TARG-19-C091

RET fusions, the most common oncogenic RET alterations, occur in approximately 1-2% of non-small cell lung cancer (NSCLC) cases and represent well-established therapeutic targets. Pralsetinib, a selective RET kinase inhibitor, has demonstrated significant efficacy and tolerability in patients with RET fusion-positive NSCLC. However, the clinical management of NSCLC with non-fusion RET structural variants remains challenging. Here, we report a case of a middle-aged male diagnosed with stage IV lung adenocarcinoma, in whom initial next-generation sequencing (NGS) revealed no actionable mutations. The patient achieved a partial response to pemetrexed and platinum-based chemotherapy, but disease progression occurred 9 months later. Upon re-biopsy, a large intragenic RET deletion involving exons 2-11 was detected. Based on this finding, the patient was treated with pralsetinib and achieved radiological tumor regression, with a progression-free survival of 5 months to date. This case highlights a potential therapeutic role for RET inhibitors even in the absence of canonical fusions, and underscores the importance of reassessing the tumor's molecular profile following treatment failure, as acquired genomic alterations may provide new targets for precision therapy.

Deregulation of the receptor tyrosine kinase RET has been implicated in medullary thyroid cancer, a small percentage of lung adenocarcinomas, endocrine-resistant breast cancer and pancreatic cancer. There are several clinically approved multi-kinase inhibitors that target RET as a secondary pharmacology but additional activities, most notably inhibition of KDR, lead to dose-limiting toxicities. There is, therefore, a clinical need for more specific RET kinase inhibitors. Herein we report our efforts towards identifying a potent and selective RET inhibitor using vandetanib 1 as the starting point for structure-based drug design. Phenolic anilinoquinazolines exemplified by 6 showed improved affinities towards RET but, unsurprisingly, suffered from high metabolic clearance. Efforts to mitigate the metabolic liability of the phenol led to the discovery that a flanking substituent not only improved the hepatocyte stability, but could also impart a significant gain in selectivity. This culminated in the identification of 36; a potent RET inhibitor with much improved selectivity against KDR.

INTRODUCTION AND OBJECTIVES: Prostate cancer is a highly heterogeneous cancer type with distinct genomic and phenotypic characteristics that drive tumorigenesis and the differential response to drug therapies. A limit number of prostate cancer cell lines and patient-derived xenograft (PDX) models hinders research to improve disease outcome. Some currently available PDX models were derived from the primary tumor samples are insufficient to recapitulate the clinical response at more advanced stages. In this study, we developed patient-derived models from patients with advanced disease and evaluated a novel AKR1C3 inhibitor in these models. METHODS: Samples received from our Pathology Biorepository Shared Resource were divided into four groups and subjected to pathological staining, RNA extraction, xenografting in NSG mice via renal capsule and subcutaneous implantation in SCID mice and conditional reprogramed cultures (CRCs) or organoid culturing. The AKR1C3 inhibitor PB was modified from celecoxib. Androgen receptor (AR), AR-V7 and AKR1C3 expression were determined by western blot. The effects of the AKR1C3 inhibitor on enzalutamide sensitivity were characterized by growth assay and colony formation assay. RESULTS: Eight PDX models have been developed from prostate cancer patients with high Gleason score and/or at the castration-resistant stages. Among the PDX models, one spontaneous indefinite cell line PS1172 was established. Early passage CRCs showed the epithelial morphology with AR positive expression. Through serially passaging PS1172 PDX with castration in SCID mice, the castration resistant cell line 1172CR was re-cultured from castration-resistant PS1172 PDX tumors. 1172CR cells were resistant to enzalutamide treatment and expressed high level of AKR1C3 and AR-V7. A novel AKR1C3 inhibitor (PB) which displayed superior potential to inhibit AKR1C3 activity and suppress enzalutamide resistant prostate cancer cell growth was tested in these models. At the same dose, PB significantly suppressed 1172CR cell growth and colony formation compared to indomethacin and enzalutamide. PB also significantly suppressed AR/AR-V7 protein expression compared to indomethacin in 1172CR cells. CONCLUSION: PS1172 and castration-resistant 1172CR cells are novel models with significant characteristics such as AR-V7 and AKR1C3. These novel prostate cancer models are ideal for small molecule testing and resistant mechanism investigating. Citation Format: Joy C. Yang, Shu Ning, Hans Adomat, Martin Gleave, Allen Gao, Christopher P. Evans, Chengfei Liu. Biological evaluation of a novel AKR1C3 inhibitor in patient-derived prostate cancer cell line and xenograft models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3099.