Prostate-specific Membrane Antigen Targeting Research Articles

Abstract 1573: Combinatorial antitumor effects of CD3-based trispecific T cell activating constructs (TriTACs) and checkpoint inhibitors in preclinical models

Abstract Tri-specific T Cell-Activating Constructs (TriTACs) are T-cell engagers that have been developed to redirect T cells to kill tumor cells. TriTAC molecules are currently being investigated in multiple phase 1/2 clinical trials in solid tumors, including HPN424 targeting prostate-specific membrane antigen (PSMA) in prostate cancer (NCT03577028), HPN536 targeting mesothelin (MSLN) in multiple malignancies (NCT03872206) and HPN328 targeting Delta Like Canonical Notch Ligand 3 (DLL3) in small cell lung cancer (SCLC) (NCT04471727). TriTAC molecules consist of a single domain antibody (sdAb) specific for a tumor antigen, a sdAb specific for serum albumin for half-life extension, and a single chain Fv (scFv) specific for the CD3ϵ subunit of the T cell receptor (TCR) complex for T cell engagement. In the presence of antigen expressing tumor cells, TriTAC treatment leads to potent T cell activation, production of proinflammatory cytokines, and re-directed tumor cell killing. Upregulation of co-stimulatory and/or inhibitory receptors during activation can potentially affect the cytolytic functions of TriTAC-activated T cells. We demonstrate that PD-1 can be readily detected on T cells subsequent to the engagement of the TCR by the TriTAC molecules HPN424, HPN536, and HPN328 in the presence of tumor cells expressing the target antigens PSMA, MSLN, and DLL3, respectively. We also demonstrate that TriTAC molecules upregulate PD-L1 on T cells in a dose-dependent manner. While induction of PD-1 on T cells suggests that cytolytic activity of TriTAC-activated T cells may be down-modulated by PD-L1-expressed by target tumor cells, simultaneous upregulation of PD-L1 on T cells suggests a second mechanism through which the functions of TriTAC-activated T cells could be further regulated by the PD1/PD-L1 axis in the tumor microenvironment through T cell-T cell interactions. In addition, in the MSLN-expressing NCI-H292 lung cancer model that co-expresses constitutive, high levels of PD-L1, both anti-PD1 and anti-PDL1 antibodies significantly enhanced the antitumor effects of the MSLN-targeting TriTAC HPN536 in vivo. Together these results demonstrate the potential utility of PD1/PDL1 blockade to enhance the potency of TriTAC-mediated tumor cell killing, supporting further investigation of these combinatorial approaches in patients. Citation Format: Mary Ellen Molloy, Laura B. Valenzuela, Priyanka E. Basak, Che-Leung Law. Combinatorial antitumor effects of CD3-based trispecific T cell activating constructs (TriTACs) and checkpoint inhibitors in preclinical models [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 1573.

64Cu-PSMA-BCH: a new radiotracer for delayed PET imaging of prostate cancer.

Develop a 64Cu labeled radiopharmaceutical targeting prostate specific membrane antigen (PSMA) and investigate its application for prostate cancer imaging. 64Cu-PSMA-BCH was prepared and investigated for stability, PSMA specificity, and micro-PET imaging. With the approval of Ethics Committee of Beijing Cancer Hospital (No. 2017KT97), PET/CT imaging in 4 patients with suspected prostate cancer was performed and the radiation dosimetry was estimated. Then, PSMA PET-ultrasound image-guided biopsies were performed on 3 patients and the fine needle aspirates were further performed for autoradiography and immunohistochemistry analysis. 64Cu-PSMA-BCH was prepared with high radiochemical yield and stability. In vivo study showed higher uptake in PSMA ( +) 22Rv1 cells than PSMA ( -) PC-3 cells (5.59 ± 0.36 and 1.97 ± 0.22 IA%/106 cells at 1h). It accumulated in 22Rv1 tumor with increasing radioactivity uptake and T/N ratios from 1 to 24h post-injection. In patients with suspected prostate cancer, SUVmax and T/N ratios increased within 24h post-injection. Compared with image at 1h post-injection, more tumor lesions were detected at 6h and 24h post-injection. The human organ radiation dosimetry showed gallbladder wall was most critical, liver and kidneys were followed, and the whole-body effective dose was 0.0292mSv/MBq. Two fine needle aspirates obtained by PET-ultrasound-guided targeted biopsy showed high radioactive signal by autoradiography, with 100% PSMA expression in cytoplasm and 30% expression in nucleus. 64Cu-PSMA-BCH was PSMA specific and showed high stability in vivo with lower uptake in liver than 64Cu-PSMA-617. Biodistribution in mice and PCa patients showed similar profile compared with other PSMA ligands and it was safe with moderate effective dosimetry. The increased tumor uptake and T/N ratios by delayed imaging may facilitate the detection of small lesions and guiding targeted biopsies.

Renal Safety of [177Lu]Lu-PSMA-617 Radioligand Therapy in Patients with Compromised Baseline Kidney Function.

Simple SummaryRadioligand therapy (RLT) targeting prostate-specific membrane antigen (PSMA) is an effective antitumor-treatment in metastatic castration-resistant prostate carcinoma (mCRPC). Concerns of potential nephrotoxicity are based on renal tubular PSMA expression and the resulting radiopharmaceutical retention during RLT, but data confirming clinically significant renal toxicity are still lacking. In this study, n = 22 patients treated within a prospective patient registry (REALITY Study) with significantly impaired baseline kidney function were investigated for treatment-associated nephrotoxicity and the potential relationship with administered activities of [177Lu]Lu-PSMA-617. As opposed to prevailing concerns, glomerular filtration rate (GFR) improved significantly in our cohort of patients and no significant correlation between change in GFR and administered activities were found. As pre-treatment chronic kidney failure did not lead to detectable RLT-induced deterioration of renal function in our study, the nephrotoxic potential of [177Lu]Lu-PSMA-617 RLT may be overestimated. We suggest not to categorically exclude patients from enrolment to PSMA-RLT due to renal impairment.Background: Radioligand therapy (RLT) targeting prostate-specific membrane antigen (PSMA) is an effective antitumor-treatment in metastatic castration-resistant prostate carcinoma (mCRPC). Concerns of potential nephrotoxicity are based on renal tubular PSMA expression and the resulting radiopharmaceutical retention during RLT, but data confirming clinically significant renal toxicity are still lacking. In this study, patients with significantly impaired baseline kidney function before initiation of therapy were investigated for treatment-associated nephrotoxicity and the potential relationship with administered activities of [177Lu]Lu-PSMA-617. Methods: Twenty-two mCRPC patients with impaired renal function (glomerular filtration rate (GFR) ≤ 60 mL/min) who received more than two cycles of [177Lu]Lu-PSMA-617 RLT (median 5 cycles and median 6-week time interval between consecutive cycles) were analyzed in this study. Patients were treated within a prospective patient registry (REALITY Study, NCT04833517). Cumulative administered activities ranged from 17.1 to 85.6 GBq with a median activity of 6.5 GBq per cycle. Renal function was closely monitored during and after PSMA-RLT. Results: Mean pre-treatment GFR was 45.0 ± 10.7 mL/min. After two (22/22 patients), four (20/22 patients), and six cycles (10/22 patients) of RLT, a significant increase of GFR was noted (each p < 0.05). End-of-treatment GFR (54.1 ± 16.7 mL/min) was significantly higher than baseline GFR (p = 0.016). Only one patient experienced deterioration of renal function (change of CTCAE grade 2 to 3). The remaining patients showed no significant reduction of GFR, including follow-up assessments (6, 9, and 12 months), and even showed improved (10/22 patients) or unchanged (11/22 patients) CTCAE-based renal impairment grades during and after the end of PSMA-RLT. No significant correlation between the change in GFR and per-cycle (p = 0.605) or cumulative (p = 0.132) administered activities were found. Conclusions: As pre-treatment chronic kidney failure did not lead to detectable RLT-induced deterioration of renal function in our study, the nephrotoxic potential of [177Lu]Lu-PSMA-617 RLT may be overestimated and not of clinical priority in the setting of palliative treatment in mCRPC. We suggest not to categorically exclude patients from enrolment to PSMA-RLT due to renal impairment.

An Improved 211At-Labeled Agent for PSMA-Targeted α-Therapy.

α-Particle emitters targeting the prostate-specific membrane antigen (PSMA) proved effective in treating patients with prostate cancer who were unresponsive to the corresponding β-particle therapy. 211At is an α-emitter that may engender less toxicity than other α-emitting agents. We synthesized a new 211At-labeled radiotracer targeting PSMA that resulted from the search for a pharmacokinetically optimized agent. Methods: A small series of 125I-labeled compounds was synthesized from tin precursors to evaluate the effect of the location of the radiohalogen within the molecule and the presence of lutetium in the chelate on biodistribution. On that basis, 211At-3-Lu was selected and evaluated in cell uptake and internalization studies, and biodistribution and PSMA-expressing (PSMA+) PC3 PIP tumor growth control were evaluated in experimental flank and metastatic (PC3-ML-Luc) models. A long-term (13-mo) toxicity study was performed for 211At-3-Lu, including tissue chemistries and histopathology. Results: The radiochemical yield of 211At-3-Lu was 17.8% ± 8.2%. Lead compound 211At-3-Lu demonstrated total uptake within PSMA+ PC3 PIP cells of 13.4 ± 0.5% of the input dose after 4 h of incubation, with little uptake in control cells. In SCID mice, 211At-3-Lu provided uptake that was 30.6 ± 4.8 percentage injected dose per gram (%ID/g) in PSMA+ PC3 PIP tumor at 1 h after injection, and this uptake decreased to 9.46 ± 0.96 %ID/g by 24 h. Tumor-to-salivary gland and tumor-to-kidney ratios were 129 ± 99 at 4 h and 130 ± 113 at 24 h, respectively. Deastatination was not significant (stomach, 0.34 ± 0.20 %ID/g at 4 h). Dose-dependent survival was demonstrated at higher doses (>1.48 MBq) in both flank and metastatic models. There was little off-target toxicity, as demonstrated by hematopoietic stability, unchanged tissue chemistries, weight gain rather than loss throughout treatment, and favorable histopathologic findings. Conclusion: Compound 211At-3-Lu or close analogs may provide limited and acceptable toxicity while retaining efficacy in management of prostate cancer.

PSMA-PET/CT Registry for Recurrent Prostate Cancer (PREP): Initial findings from a single center.

5064 Background: Several lesion-targeted therapies exist for locally recurrent or limited stage metastatic prostate cancer (PCa) post-radiotherapy (RT) and radical prostatectomy (RP). However, detection of disease sites is limited using conventional imaging (CI) including computed tomography (CT) and bone scan. Prostate specific membrane antigen (PSMA) targeting PET radiopharmaceuticals like [18F]DCFPyL may help detect disease not seen on CI. Our objective was to assess the ability of PSMA targeted PET/CT to detect sites of disease recurrence and impact on patient management. Methods: This multi-center prospective registry study included six Ontario centers. Eligible patients in 1 of 7 clinical cohorts (Table) were identified and approved by Cancer Care Ontario (CCO) to have restaging with PSMA targeted PET/CT. Referring physicians were asked to complete a form indicating whether a change in management strategy would occur based on the PET/CT results. At 6 months post-PET/CT, actual patient management will be confirmed via provincial registries. These interim results are from a single center. Results: 253 patients were enrolled and had a PSMA targeted PET/CT. At baseline, median age was 71 years (range 50-102 years) and median PSA was 2.7 ng/mL (range 0.04-134.0 ng/mL). The majority of patients (n=59; 23.3%) were in cohort 2 (biochemical failure post-RP). In patients with negative CI, PSMA targeted PET/CT detected disease sites in 68.5% (170/248), resulting in a change in management for 67.8% (137/202) overall and 72.1% and 64.3% post-RT and post-RP, respectively. Conclusions: PSMA targeted PET/CT detected occult lesions on CI in the majority of patients enrolled, leading to a high rate of change in management. Our institutional results are in keeping with preliminary results reported for the provincial cohort. Clinical trial information: NCT03718260. [Table: see text]

Phase I study of CCW702, a bispecific small molecule-antibody conjugate targeting PSMA and CD3 in patients with metastatic castration-resistant prostate cancer (mCRPC).

TPS5094 Background: CCW702 is a novel bispecific antibody comprised of a small molecule imaging agent ligand (DUPA) with specificity for prostate specific membrane antigen (PSMA) conjugated to an anti-CD3 antibody via an unnatural amino acid. This format has the structure of an antibody drug conjugate with the activity of a CD3-engaging bispecific antibody. The design of CCW702 was leveraged to optimize the structure and function of T cell redirected cytotoxicity against PSMA-positive prostate cancer tumors in preclinical development. Methods: This is a first-in-human, open-label, multi-center phase 1 study evaluating the safety and tolerability of CCW702 when administered via subcutaneous (SC) injection in men with mCPRC. This study will be conducted in two parts: Part I, a dose escalation to determine the maximum tolerated dose (MTD) and recommended phase 2 dose (R2PD); Part II, a dose expansion to determine efficacy at the R2PD. Safety, pharmacokinetics (PK), pharmacodynamics (PD), immunogenicity, and preliminary efficacy will be evaluated. Efficacy will be assessed by change in circulating tumor cells (CTC), PSA50 response rate, and objective tumor response by RECIST v1.1. Key biomarkers include characterization of CTC, T cell phenotyping in peripheral blood, chemokines and cytokines over time, and evaluation of available tumor biopsies by IHC. Key inclusion criteria include men age ≥ 18 years with histologically or cytologically confirmed adenocarcinoma who, in the metastatic setting, have progressed on at least one novel AR-targeted therapy. Up to 1 prior chemotherapy regimen is allowed. This study will enroll 20-30 patients in Part 1 and approximately 40 patients in Part 2. The study opened in December 2019 and is currently enrolling in the dose escalation phase. Clinical trial information: NCT04077021.

More Than Meets the Eye: Scientific Rationale behind Molecular Imaging and Therapeutic Targeting of Prostate-Specific Membrane Antigen (PSMA) in Metastatic Prostate Cancer and Beyond.

Simple SummaryProstate-specific membrane antigen (PSMA) is a transmembrane protein that is overexpressed in prostate cancer and correlates with the aggressiveness of the disease. PSMA is a promising target for imaging and therapeutics in prostate cancer patients validated in prospective trials. However, the role of PSMA in prostate cancer progression is poorly understood. In this review, we discuss the biology and scientific rationale behind the use of PSMA and other targets in the detection and theranostics of metastatic prostate cancer.Prostate cancer is the second most common cancer type in men globally. Although the prognosis for localized prostate cancer is good, no curative treatments are available for metastatic disease. Better diagnostic methods could help target therapies and improve the outcome. Prostate-specific membrane antigen (PSMA) is a transmembrane glycoprotein that is overexpressed on malignant prostate tumor cells and correlates with the aggressiveness of the disease. PSMA is a clinically validated target for positron emission tomography (PET) imaging-based diagnostics in prostate cancer, and during recent years several therapeutics have been developed based on PSMA expression and activity. The expression of PSMA in prostate cancer can be very heterogeneous and some metastases are negative for PSMA. Determinants that dictate clinical responses to PSMA-targeting therapeutics are not well known. Moreover, it is not clear how to manipulate PSMA expression for therapeutic purposes and develop rational treatment combinations. A deeper understanding of the biology behind the use of PSMA would help the development of theranostics with radiolabeled compounds and other PSMA-based therapeutic approaches. Along with PSMA several other targets have also been evaluated or are currently under investigation in preclinical or clinical settings in prostate cancer. Here we critically elaborate the biology and scientific rationale behind the use of PSMA and other targets in the detection and therapeutic targeting of metastatic prostate cancer.

Fully human antibody VH domains to generate mono and bispecific CAR to target solid tumors

BackgroundChimeric antigen receptor (CAR) T cells are effective in B-cell malignancies. However, heterogeneous antigen expression and antigen loss remain important limitations of targeted immunotherapy in solid tumors. Therefore, targeting multiple...

Preclinical Evaluation of a Fluorine-18 Labeled Probe for the Detection of the Expression of PSMA Level in Cancer.

Prostate-specific membrane antigen (PSMA) is a prospect biomarker for the treatment of prostate cancer. Meanwhile, positron emission tomography (PET) is being developed as a significant imaging modality in cancer diagnosis. A new PET probe Glu-ureido-Lys-naphthylalanine-tranexamic acid-Gly(AMBF3)-triiodobenzoic acid (18F-GLNTGT) was radiosynthesized by a one-step 18F-labeled method. 18F-GLNTGT was obtained with a radioactivity yield (RCY) of 12.16 ± 6.4% and a good radiochemical purity (RCP > 96%). The cell uptakes of 18F-GLNTGT were determined to be 15.9 ± 0.43% ID and 9.47 ± 1.26% ID at 15 min in LNCaP cells and PC-3 cells, respectively. The cell internalization of 18F-GLNTGT was determined to be 12.89 ± 0.94% ID and 5.34 ± 0.15% ID at 15 min in LNCaP cells and PC-3 cells, respectively. It is suggested that the probe has good specificity targeting PSMA. From the results of 18F-GLNTGT binding affinity with PSMA, it has a higher affinity and a Ki value of 0.49 nM (95% confidence interval (CI): 0.35–0.67 nM). In PET imaging, 18F-GLNTGT showed the highest tumor uptake of 3.51 ± 0.15% ID/g at 45 min and the maximum tumor/muscle (T/Mmax) ratio of 3.68 ± 0.29 at 60 min post-injection (p.i.) in LNCaP tumors. The control probe 18F-AlF-NOTA-RGD2 presented the highest tumor uptake of 4.2 ± 0.54% ID/g at 7.5 min and the T/Mmax ratio of 2.72 ± 0.63 at 45 min p.i. in LNCaP tumors. The results showed that the probe has a higher tumor/muscle ratio compared with the control probe 18F-AlF-NOTA-RGD2. Although the probe 18F-GLNTGT has some limitations for CT signal detection both in cells and in vivo, it is still a promising PET probe for targeting PSMA membrane protein.

Incorporation of Unnatural Amino Acid into Antibody Fragment for Creating a Stable Antibody Drug Conjugate

The traditional chemotherapy drug has been used as a standard cancer treatment, however it has resulted a modest survival benefit and damaged non-cancerous cells. Thus, the novel strategies which can improve selectivity and specificity in chemotherapy are urgently needed. Antibody drug conjugate (ADC) combines monoclonal antibody and cytotoxic drug is a potential regimen as targeted therapy. However, the heterogeneous mixtures has been observed using the current ADC methods. Here, we develop the strategy for generation a stable ADC utilising modified single chain antibody fragment (scFv) containing azide group for click chemistry reaction with alkyne containing cytotoxic drug. This research focused on targeting prostate cancer as a model disease utilising targeting prostate specific membrane antigen (PSMA) receptor which is overexpressed in all prostate cancer stages. The unnatural amino acid para-azido phenyl alanine (pAzF) has been successfully incorporated into anti-PSMA J591 scFv and specifically bound and internalised into PSMA positive cancer cells. This mutant scFv were also successfully conjugated into a linker containing cyclo-alkyne, DBCO-PEG4-DBCO as a model for creating ADC through copper-free click chemistry reaction. This bioconjugation method is promising as a versatile strategy for generating a stable ADC to improve therapeutic efficacy in cancer treatment.

Pretherapeutic first-in-human targeting trial of anti-hK2 antibody h11B6: A potential molecular vector for alpha-particle radioimmunotherapy.

122 Background: Radioligand therapy for metastatic castration resistant prostate cancer (mCRPC) has focused to date on targeting prostate specific membrane antigen, an approach associated with salivary gland toxicity and variable lesion expression. New targets are needed. Human kallikrein-2 (hK2) is overexpressed in most prostate cancer, with minimal normal tissue expression or serum secretion. Humanized monoclonal antibody (mAb) h11B6 exclusively targets hK2 in pre-clinical studies. This phase 0 study (NCT04116164) is a first-in-human trial of [111In]-DOTA-h11B6 to determine the hk2-targeting potential of h11B6. Objective: The purpose of this ongoing protocol is to characterize the biodistribution of [111In]-DOTA-h11B6 and determine a suitable mAb mass that targets mCRPC with minimal non-tumor targeting. Methods: Patients with progressive mCRPC were enrolled. A single slow bolus infusion of 2 mg [111In]-DOTA-h11B6 (nominally 185 MBq [111In]), was administered IV with or without 8 mg h11B6. Patients were observed for adverse events (AE) for at least 2 weeks. Serial gamma camera imaging including at least one SPECT/CT scan was performed up to 8 days post-administration. Serial blood samples were obtained over 2 weeks to determine serum radioactivity and h11B6 protein levels. Dosimetry for normal organs was estimated using OLINDA-EXM. Results: Results for the first 6 patients are summarized in Table. Treatment was tolerated in all patients with no AEs and no evidence of enhanced accumulation in any organ including salivary glands. Initial volume of distribution appeared confined to the vascular compartment. Slow clearance of radioactivity from the vascular compartment was observed with gradual targeting to skeletal and non-skeletal lesions in all patients. Conclusions: h11B6 is a novel mAb that targets hK2, localizes to bone and soft tissue metastases, has no significant normal tissue uptake, and spares salivary glands. An IND for a phase I therapeutic study using [225Ac] has been secured. Clinical trial information: NCT04116164. [Table: see text]

Study of the dosimetry, safety, and potential benefit of 177Lu-PSMA-617 radionuclide therapy prior to radical prostatectomy in men with high-risk localized prostate cancer (LuTectomy study).

TPS264 Background: High-risk localised prostate cancer (HRCaP) is treated by radical prostatectomy (RP) or radiotherapy. Despite curative intent, a significant number of men will progress with metastatic disease or local recurrence. Lutetium-177 radiolabelled to the small molecule PSMA-617 targeting prostate-specific membrane antigen (Lu-PSMA) has proven efficacious in men with metastatic castration-resistant prostate cancer who have progressed after standard-of-care. The LuTectomy trial evaluates whether administration of Lu-PSMA before radical prostatectomy in men with HRCaP will deliver high doses of radiation to the prostate and involved lymph nodes. It also aims to assess the feasibility, safety profile and oncological efficacy of Lu-PSMA. Methods: Lutectomy is an open-label, phase I/II non-randomised clinical trial. 20 men with HRCaP defined by European Association of Urology who are scheduled for RP and pelvic lymph node dissection (PLND) will be recruited. All men will have high PSMA-avidity with a standardised uptake value (SUVmax) of ≥ 20 on 68Ga-PSMA PET/CT. The initial 10 participants will receive one cycle of 5GBq Lu-PSMA intravenously and the latter 10 men will receive two cycles of 5GBq Lu-PSMA per cycle six weeks apart. RP with PLND will be performed six weeks later. Participants will be followed up for three years. The primary outcome is to determine the radiation absorbed dose in the prostate and involved lymph nodes. Three-time point quantitative single-photon emission computed tomography (SPECT/CT) will be used to estimate radiation dosimetry using a voxelated technique incorporating partial volume correction. Marrow absorbed dose will also be evaluated using serial blood measures to model pharmacokinetic clearance with a multi-phase exponential model. Translational research samples will include the original biopsy, prostatectomy specimen and plasma/whole blood samples. Secondary objectives include evaluation of the PSMA PET/CT imaging response (defined by SUVmax decline &gt;30%), PSA response, pathological response following Lu-PSMA, adverse effects of Lu-PSMA, surgical safety, and health-related quality of life. Post estimates for time-to-event endpoints will be estimated using the Kaplan-Meier method. The first patient was recruited in July 2020 and recruitment is expected to take up to two years. ClinicalTrials.gov Identifier: NCT04430192. Funding: Movember and Medical Research Future Fund (MRFF), Endocyte Inc., a Novartis Company, E.J. Whitten Foundation. Clinical trial information: NCT04430192.

Small Molecule-Based Prodrug Targeting Prostate Specific Membrane Antigen for the Treatment of Prostate Cancer.

Metastatic castration-resistant prostate cancer poses a serious clinical problem with poor outcomes and remains a deadly disease. New targeted treatment options are urgently needed. PSMA is highly expressed in prostate cancer and has been an attractive biomarker for the treatment of prostate cancer. In this study, we explored the feasibility of targeted delivery of an antimitotic drug, monomethyl auristatin E (MMAE), to tumor tissue using a small-molecule based PSMA lig-and. With the aid of Cy5.5, we found that a cleavable linker is vital for the antitumor activity of the ligand-drug conjugate and have developed a new PSMA-targeting prodrug, PSMA-1-VcMMAE. In in vitro studies, PSMA-1-VcMMAE was 48-fold more potent in killing PSMA-positive PC3pip cells than killing PSMA-negative PC3flu cells. In in vivo studies, PSMA-1-VcMMAE significantly inhibited tumor growth leading to prolonged animal survival in different animal models, including metastatic prostate cancer models. Compared to anti-PSMA antibody-MMAE conjugate (PSMA-ADC) and MMAE, PSMA-1-VcMMAE had over a 10-fold improved maximum tolerated dose, resulting in improved therapeutic index. The small molecule-drug conjugates reported here can be easily synthesized and are more cost efficient than anti-body-drug conjugates. The therapeutic profile of the PSMA-1-VcMMAE encourages further clin-ical development for the treatment of advanced prostate cancer.

Detection of Early Progression with 18F-DCFPyL PET/CT in Men with Metastatic Castration-Resistant Prostate Cancer Receiving Bipolar Androgen Therapy.

Bipolar androgen therapy (BAT) is an emerging treatment for metastatic castration-resistant prostate cancer (mCRPC). 18F-DCFPyL is a small-molecule PET radiotracer targeting prostate-specific membrane antigen (PSMA). We analyzed the utility of 18F-DCFPyL PET/CT in determining clinical response to BAT. Methods: Six men with mCRPC receiving BAT were imaged with 18F-DCFPyL PET/CT at baseline and after 3 mo of treatment. Progression by PSMA-targeted PET/CT was defined as the appearance of any new 18F-DCFPyL-avid lesion. Results: Three of 6 (50%) patients had progression on 18F-DCFPyL PET/CT. All 3 had stable disease or better on contemporaneous conventional imaging. Radiographic progression on CT or bone scanning was observed within 3 mo of progression on 18F-DCFPyL PET/CT. For the 3 patients who did not have progression on 18F-DCFPyL PET/CT, radiographic progression was not observed for at least 6 mo. Conclusion: New radiotracer-avid lesions on 18F-DCFPyL PET/CT in men with mCRPC undergoing BAT can indicate early progression.

PSMA targeting in metastatic castration-resistant prostate cancer: where are we and where are we going?

Prostate-specific membrane antigen (PSMA) is highly expressed on the membrane of most prostate cancer cells and to a lesser extent in normal tissues. Many vectors targeting this protein have been created over the past decade and numerous clinical studies have positively demonstrated the tolerance and efficacy of radiolabeled prostate-specific membrane antigen ligands for PSMA radioligand therapy (PRLT). Preliminary results are encouraging that PRLT will become an important addition to the current therapeutic options in a number of settings. Improvement in radiopharmaceutical targeting and combination with other oncological agents are under investigation to further improve its therapeutic efficacy. These encouraging results have led to the development of other therapies using PSMA as a target, such as PSMA–targeted chimeric antigen receptor T-cells, PSMA–targeted antibody drug conjugates, and PSMA–targeted bi-specific T-cell-directed therapy. This narrative review details the current state and advancements in prostate-specific membrane antigen targeting in prostate cancer treatment.

Methods to Validate Binding and Kinetics of "Proximity-Inducing" Covalent Immune-Recruiting Molecules.

The emergence of covalent inhibitors and chemoproteomic probes in translational chemical biology research requires the development of robust biophysical and analytical methods to characterize their complex interactions with target biomolecules. Importantly, these methods must efficiently assess target selectivity and accurately discern noncovalent binding from the formation of resultant covalent adducts. One recently reported covalent chemical tool used in tumor immune oncology, covalent immune recruiters (CIRs), increases the proximity of immune cells and cancer cells, promoting immune recognition and response. Herein we describe biolayer interferometry (BLI) biosensor, flow cytometry, and solution fluorescence-based assay approaches to characterize CIR:antibody binding and CIR-antibody covalent-labeling kinetics. BLI technology, akin to surface plasmon resonance, provides the unique opportunity to investigate molecular binding and labeling kinetics both on a solid surface (Basic Protocol 1) and in solution (Alternate Protocol 1). Here, recruitment of mass-containing proteins to the BLI probe via CIR is measured with high sensitivity and is used as a readout of CIR labeling activity. Further, CIR technology is used to label antibodies with a fluorescent handle. In this system, labeling is monitored via SDS-PAGE with a fluorescence gel imager, where increased fluorescence intensity of a sample reflects increased labeling (Basic Protocol 2). Analysis of CIR:antibody target-specific immune activation is demonstrated with a flow cytometry-based antibody-dependent cellular phagocytosis (ADCP) assay (Basic Protocol 3). This ADCP protocol may be further used to discern CIR:antibody binding from covalent adduct formation (Alternate Protocol 3). For the protocols described, each method may be used to analyze characteristics of any covalent-tagging or antibody-recruiting small molecule or protein-based technology. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Determining "on-probe" reaction kinetics of CIR1/CIR4 via biolayer interferometry with Octet RED96 Alternate Protocol 1: Determining "in-solution" reaction kinetics of prostate-specific membrane antigen targeting CIR (CIR3) via biolayer interferometry with Octet RED96 Basic Protocol 2: Reaction kinetics of covalently labeled antibodies via fluorescence SDS-PAGE Basic Protocol 3: Small molecule-directed antibody-dependent cellular phagocytosis on live human cells measured via flow cytometry Alternate Protocol 2: Kinetic analysis of CIR3:antibody labeling via antibody-dependent cellular phagocytosis on flow cytometry Support Protocol 1: Activation of U937 monocytes with interferon γ Support Protocol 2: Labeling streptavidin beads with biotinylated prostate-specific membrane antigen receptor.

Is Less More? Influence of the Coordination Geometry of Copper(II) Picolinate Chelate Complexes on Metabolic Stability.

A growing number of copper(II) complexes have been identified as suitable candidates for biomedical applications. Here, we show that the biocompatibility and stability of copper(II) complexes can be tuned by directed ligand design and complex geometry. We demonstrate that azamacrocycle-based chelators that envelope copper(II) in a five-coordinate, distorted trigonal-bipyramidal structure are more chemically inert to redox-mediated structural changes than their six-coordinate, Jahn-Teller-distorted counterparts, as evidenced by electrochemical, crystallographic, electron paramagnetic resonance, and density functional theory studies. We further validated our hypothesis of enhanced inertness in vitro and in vivo by employing Cu-64 radiolabeling of bifunctional analogues appended to a prostate-specific membrane antigen targeting dipeptide. The corresponding Cu-64 complexes were tested for stability in vitro and in vivo, with the five-coordinate system demonstrating the greatest metabolic stability among the studied picolinate complex series.

Abstract 2779: Non-invasive quantification of pharmacodynamic effects of an antibody targeting PSMA by PET imaging

Abstract Introduction: Prostate-specific membrane antigen (PSMA) is highly expressed in prostate cancers (PCa) and a target for the development of a variety of therapeutics. Knowledge of how those therapeutics engage tumor PSMA could improve dose finding and optimization. Here we studied the pharmacodynamic effects of a PSMA monoclonal antibody (5D3, mAb) on target engagement and accessible levels of PSMA in the tumors in PCa and melanoma xenograft models known to express PSMA. We also sought to evaluate effect of tumor burden on the pharmacodynamic effects of the mAb and accessible tumor PSMA levels. To quantify accessible PSMA levels, we used [18F]DCFPyL, an established PSMA binding positron emission tomography (PET) imaging agent. Methods: Prostate (22RV1) and skin (SKMEL3) cancer cells were tested for PSMA expression by flow cytometry. Corresponding tumors were generated in the upper flanks of male NSG mice and used for PET imaging and biodistribution studies. [18F]DCFPyL was synthesized using established procedures. To assess the effect of 5D3 dose on accessible PSMA levels, mice with 22RV1 tumors were treated with a single dose of 0.3, 1, 3, 10 and 30 mg/kg of mAb or saline as a control. Mice with one and three 22RV1 tumors were used to establish low and high tumor burden models, respectively and treated with a single 1 and 10 mg/kg dose of 5D3. All the treatments were performed for 24h. After 5D3 treatment, [18F]DCFPyL was injected and imaging and biodistribution studies were performed at 1 hour after radiotracer injection. All the data were normalized and presented as % of injected dose per gram (%ID/g). Results: We observed a negative correlation between [18F]DCFPyL uptake in tumors and antibody dose, indicating a decrease in accessible tumor PSMA levels with increased mAb dose. Ex vivo biodistribution studies showed high [18F]DCFPyL uptake (13.2±1.4 %ID/g) in saline treated controls and a 25, 46 and 76% reduction in mice treated with 0.3, 1 and 10 mg/kg dose of mAb, respectively. Those findings were validated in SKMEL3 tumors with 4-fold lower PSMA expression. Collectively, these data establish that accessible PSMA levels in the tumors are reduced for [18F]DCFPyL binding with increased 5D3 dose. Next, we studied the effect of tumor burden on antibody dose and accessible PSMA levels at the tumor. In mice bearing one 22RV1 tumor (low tumor burden), [18F]DCFPyL uptake in tumors was reduced by 52% and 77% in mice treated with 1 and 10 mg/kg dose of 5D3, respectively. In mice with three tumors (high tumor burden), [18F]DCFPyL uptake in tumors was reduced by 29% and 56% at 1 and 10 mg/kg dose of 5D3, respectively. No differences were observed in saline treated controls between mice with one or three tumors. These data suggest higher accessible PSMA levels in mice with three tumors compared to mice with one tumor when treated with the same dose of an mAb. These observations indicate that high tumor burden leads to increased antibody processing and accessible PSMA levels in tumors, which could have implication for dosing of PSMA targeted therapeutics. Conclusion: Quantification of accessible PSMA levels, measured using [18F]DCFPyL uptake, can be used as a tool for dose-finding and optimization of PSMA targeted therapeutics. Citation Format: Akhilesh Mishra, Zora Nováková, Ala Lisok, Dhiraj Kumar, Ronnie C. Mease, Cyril Bařinka, Sridhar Nimmagadda. Non-invasive quantification of pharmacodynamic effects of an antibody targeting PSMA by PET imaging [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2779.

Heterodimeric Radiotracer Targeting PSMA and GRPR for Imaging of Prostate Cancer-Optimization of the Affinity towards PSMA by Linker Modification in Murine Model.

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) are promising targets for molecular imaging of prostate cancer (PCa) lesions. Due to the heterogenic overexpression of PSMA and GRPR in PCa, a heterodimeric radiotracer with the ability to bind to both targets could be beneficial. Recently, our group reported the novel heterodimer BQ7800 consisting of a urea-based PSMA inhibitor, the peptide-based GRPR antagonist RM26 and NOTA chelator. The study reported herein, aimed to improve the affinity of BQ7800 towards PSMA by changing the composition of the two linkers connecting the PSMA- and GRPR-targeting motifs. Three novel heterodimeric analogues were synthesized by incorporation of phenylalanine in the functional linker of the PSMA-binding motif and/or shortening the PEG-linker coupled to RM26. The heterodimers were labeled with indium-111 and evaluated in vitro. In the competitive binding assay, BQ7812, featuring phenylalanine and shorter PEG-linker, demonstrated a nine-fold improved affinity towards PSMA. In the in vivo biodistribution study of [111In]In-BQ7812 in PC3-pip tumor-bearing mice (PSMA and GRPR positive), the activity uptake was two-fold higher in the tumor and three-fold higher in kidneys than for [111In]In-BQ7800. Herein, we showed that the affinity of a bispecific PSMA/GRPR heterodimer towards PSMA could be improved by linker modification.

Nanobodies Targeting Prostate-Specific Membrane Antigen for the Imaging and Therapy of Prostate Cancer.

The repertoire of methods for the detection and chemotherapeutic treatment of prostate cancer (PCa) is currently limited. Prostate-specific membrane antigen (PSMA) is overexpressed in PCa tumors and can be exploited for both imaging and drug delivery. We developed and characterized four nanobodies that present tight and specific binding and internalization into PSMA+ cells and that accumulate specifically in PSMA+ tumors. We then conjugated one of these nanobodies to the cytotoxic drug doxorubicin, and we show that the conjugate internalizes specifically into PSMA+ cells, where the drug is released and induces cytotoxic activity. In vivo studies show that the extent of tumor growth inhibition is similar when mice are treated with commercial doxorubicin and with a 42-fold lower amount of the nanobody-conjugated doxorubicin, attesting to the efficacy of the conjugated drug. These data highlight nanobodies as promising agents for the imaging of PCa tumors and for the targeted delivery of chemotherapeutic drugs.