Antibody J591 Research Articles

Prostate-Specific Membrane Antigen: Gateway to Management of Advanced Prostate Cancer.

Prostate-specific membrane antigen (PSMA) as a transmembrane protein is overexpressed by prostate cancer (PC) cells and is accessible for binding antibodies or low-molecular-weight radioligands due to its extracellular portion. Successful targeting of PSMA began with the development of humanized J591 antibody. Due to their faster clearance compared to antibodies, small-molecule radioligands for targeted imaging and therapy of PC have been favored in recent development efforts. PSMA positron emission tomography (PET) imaging has higher diagnostic performance than conventional imaging for initial staging of high-risk PC and biochemical recurrence detection/localization. However, it remains to be demonstrated how to integrate PSMA PET imaging for therapy response assessment and as an outcome endpoint measure in clinical trials. With the recent approval of 177Lu-PSMA-617 by the US Food and Drug Administration for metastatic castration-resistant PC progressing after chemotherapy, the high value of PSMA-targeted therapy was confirmed. Compared to standard of care, PSMA-based radioligand therapy led to a better outcome and a higher quality of life. This review, focusing on the advanced PC setting, provides an overview of different approved and nonapproved PSMA-targeted imaging and therapeutic modalities and discusses the future of PSMA-targeted theranostics, also with an outlook on non-radiopharmaceutical-based PSMA-targeted therapies.

Randomized, double-blinded phase II study of ketoconazole (keto), hydrocortisone (HC), and anti-PSMA antibody J591 labeled with 177Lu or 111In in patients (pts) with high-risk non-metastatic (met) castration-resistant prostate cancer (M0 CRPC).

LBA21 Background: Up to 1/3 of pts develop biochemical relapse following primary therapy. Many are not cured with salvage local therapy, likely because of undetectable distant disease. PSMA is expressed on most PC and can be targeted by radiolabeled J591. 177Lu is a predominantly β-emitting radionuclide and also has γ emission which allows imaging. 111In is predominantly a γ emitter, also with some auger emission for therapy. Hormonal therapy is effective and may increase PSMA expression and radiosensitize. In this DOD-funded study initiated in the pre-PSMA PET and AR signaling inhibitor era, we hypothesized that 177Lu prolongs 18-month (mo) met-free survival (MFS) more than 111In in pts with high risk, M0 CRPC when targeting PSMA via J591 in combo with keto and HC. Methods: Pts with high-risk M0 CRPC defined by PSA DT < 8 mo and/or absolute PSA > 20 ng/mL and serum testosterone < 50 ng/mL with no evidence of metastatic disease on CT/MRI and bone scan were eligible. Treatment included a minimum 4 week lead-in with keto 400 mg TID and HC 20 mg AM, 10 mg PM (both of which could be continued until unacceptable toxicity or development of mets) and a single infusion of J591 with 2:1 randomization to 177Lu (70 mCi/m2) or 111In (5 mCi) in double-blinded fashion. The final version of the protocol was designed to randomize 55 pts for 80% power to detect a difference in 18-mo MFS with one-sided alpha of 10%. Secondary endpoints include median MFS, PSA response, overall survival, and toxicity. Results: 55 pts with median age 68 (range 52 - 88), 75% prostatectomy, 23% primary radiation, 2% primary ADT; 19% local salvage therapy. Median PSA doubling time 3 mo (range 0.87 – 7.85), median baseline PSA 8.0 (range 1-78). In intent to treat analysis (5 without imaging and 4 lost to follow up by 18 mo), 50% developed mets by 18 mo with 177Lu vs 76% with 111In (p=0.066). Median MFS was 23.8 mo vs 20.8 mo, and biochemical PFS was 18.67 vs 8.87 mo, favoring 177Lu in analyses censoring start of new treatment. Confirmed >50% PSA decline occurred in 82% with 177Lu and 71% with 111In. Grade >3 heme AEs were more common with 177Lu vs 111In, including neutropenia (57% vs 11%, with 1 febrile neutropenia) and thrombocytopenia (77% vs 11%, with 25% vs 6% platelet transfusions), whereas Gr >3 non-heme AEs were less common with 177Lu vs 111In, including abdominal pain (0 vs 11%), ALT increase (3.3% vs 22%), and diarrhea (0 vs 22%). Conclusions: Anti-PSMA mAb J591 with keto/HC when radiolabeled with 177Lu leads to improved 18-month met-free survival vs 111In. Most pts had significant PSA decline with either version of radiolabeled J591 with keto/HC. Hematologic toxicity is more common with 177Lu. This supports the development of anti-PSMA radioimmunotherapy for low volume advanced PC, though the optimal radionuclide and targeting agent is unknown. Clinical trial information: NCT00859781 .

Antibodies targeting Prostate-Specific Membrane Antigen positive prostate cancer: from diagnostic imaging to theranostics.

Targeting Prostate-Specific Membrane Antigen (PSMA) has paved the way for personalized medicine in prostate cancer (PCa) patients. This review aims to highlight the role of PSMA targeting antibodies in PCa, for diagnostic and therapeutic purposes. PSMA Positron Emission Tomography/Computed Tomography has been a game changer in the diagnosis of PCa in the recent decade. Two anti-PSMA monoclonal antibodies have been studied in PCa: 7E11-C35 (limited use) and J591. J591 antibody was used for diagnostic purposes coupled with different radionuclides. Most importantly, it was combined to numerous therapeutic radionuclides such as Lutetium-177 (177Lu), Yttrium-90 (90Y), Indium-111 (111In), and Actinium-225 (225Ac). It was also conjugated to drugs forming antibody-drug conjugates (e.g. MLN2704 and PSMA-ADC). These compounds were tested in recent phase I/II clinical trials. PSMA targeting antibodies are very promising for further clinical investigation and continue to be a momentous research area, for both imaging and therapeutic settings. Although some clinical trials resulted in unfavorably safety profiles for some antibodies, they validated PSMA as a crucial immunoconjugate target.

Phase I study of 225Ac-J591 for men with metastatic castration-resistant prostate cancer (mCRPC).

5015 Background: Antibodies and small molecule ligands target PSMA with different kinetics and biodistribution, with certain sites of PSMA expression such as salivary/lacrimal glands, kidneys, and small bowel less accessible to large antibodies. Alpha emitters such as 225Ac have high potency, but short range. We report dose-escalation plus expansion cohort results of a first in human study of 225Ac-J591. Methods: Men with progressive mCRPC following at least 1 potent AR-pathway inhibitor (ARPI, e.g. abi/enza) and chemo (or unfit/refuse chemo) without limit of # prior therapies (including Ra-223 or prior 177Lu-PSMA) provided adequate organ function were eligible. Baseline 68Ga-PSMA11 PET was performed, but not used for eligibility. Dose-escalation was in single-subjects x4 followed by 3+3 with a single infusion of 225Ac-J591 (13.3 KBq/kg with planned escalation up to 93.3 KBq/kg). Dose-limiting toxicity (DLT) was defined as attributable grade (Gr) 4 heme toxicity or Gr 3/4 non-heme tox. Imaging, genomic, patient-reported outcomes (PRO), and immune correlates embedded. Results: 32 men were treated with a single dose of 225Ac-J591 on 7 dose levels with expansion at the highest dose level (n = 16). Median age 69.5 (range 52-89), PSA 149.1 (4.8-7168.4); 75% with >2 prior ARPI, 62.5% chemo, 28% Ra-223, 43.7% 177Lu-PSMA. One (3.1%) CALGB (Halabi) good prognostic risk, 8 (25%) intermediate, and 23 (71.9%) poor risk. While PSMA uptake was not a prerequisite for treatment, of 28 with pre-treatment PSMA PET, none had tumor SUVmax < liver, 5 (17.8%) with tumor SUVmax 1-2.5x liver, 2 (7.2%) with tumor SUVmax 2.5-5x liver, and 21 (75%) with tumor SUVmax > 5x liver SUVmean. 1 of 6 in cohort 6 (80 KBq/kg) had DLT (Gr 4 anemia and platelets) with 0 of 6 at the highest dose level (93.3 KBq/Kg) and this dose was expanded. High Gr AEs were restricted to hematologic: In addition to DLT, 4 (12.5%) Gr 3 platelets and 2 (6.2%) with Gr 3 neutropenia. Non-heme AE’s were restricted to Gr 1/2 and included: 10 (31.2%) fatigue, 5 (15.6%) pain flare, 14 (43.7%) nausea, 8 (25%) with Gr 1 xerostomia (of which 5 received prior 177Lu-PSMA), 12 (37.5%) AST elevation. Despite prior treatment including 177Lu-PSMA and no selection for PSMA expression, 22 (68.7%) with any PSA decline, 12 (37.5%) with > 50% PSA decline. Of 21 with paired baseline and 12-wk CTC counts, 12 declined (5 converting from unfavorable to favorable and 5 converting detectable to 0), 5 remained 0, 4 increased. In the subset with PRO data, pain scores by BPI-SF tended to improve by wk 12. Following a single dose of 225Ac-J591, median PFS 7.2 months [95% CI 4.6-NR], median OS 10.9 months [7.6-21.1]. Conclusions: PSMA-targeted alpha-emitter 225Ac utilizing intact antibody J591 is tolerable with early evidence of clinical activity. Based upon these results, a follow up study [NCT04506567] testing multiple and fractionated dosing of 225Ac-J591 is underway. Clinical trial information: NCT03276572.

Pilot study of anti-prostate-specific membrane antigen (PSMA) antibody J591 for men with metastatic castration-resistant prostate cancer (mCRPC) and unfavorable circulating tumor cell (CTC) count.

120 Background: Elevated CTC counts are associated with a poor prognosis in men with mCRPC. PSMA targeted radionuclide therapy has been associated with decline in CTC count, but it remains unclear whether this effect results from radionuclide-induced cytotoxicity. J591 was engineered to have antibody-dependent cytotoxicity. A subset of patients was observed to have CTC count decline following imaging with 111In-J591, so a prospective study was launched to test the hypothesis that “naked” J591 leads to CTC count decline. Methods: In a Simon 2-stage dose de-escalation study, men with progressive mCRPC and unfavorable CTC count (CellSearch > 4) received a single dose of J591. Initial dose cohort 300 mg with de-escalation to 20 mg. CTC count was re-assessed 4, 8, and 12 weeks following therapy along with PSA and standard imaging. An optional PSMA PET was included prior to treatment. The primary endpoint was proportion of subjects with conversion to favorable CTC count ( < 5 CTCs/7.5 mL blood) and/or > 30% decline from baseline within 12 weeks post-treatment. Results: 10 men were enrolled, 9 of whom were evaluable (1 died of progressive mCRPC prior to post-treatment CTC count). Median age was 71.5 years (range 60-81), 78% had prior chemo, ECOG PS 1 in 45% and 2 in 55%. 7 of 9 (78%) evaluable subjects were Halabi CALGB prognostic poor risk category and 2 (22%) intermediate. 6 of 9 had pre-treatment PSMA PET/CT (three 89Zr-J591 and three 68Ga-PSMA11). Though not required, all scans showed > 1 lesion with SUVmax > liver SUV (range 9.12-70.15). 2 of 6 in the 300 mg cohort had CTC count decline; 1 of 6 converted to favorable count (9 to 0 with decrease of 35 to 12 in other). 3 were treated with 20 mg; 1 had CTC count decline of 316 to 112, but 0 converted to favorable count. Across both cohorts, 3 of 9 had a CTC count decline at any point in time, ranging from 65-100% decline. With the pre-specified 2-stage design, enrollment was halted for futility based upon the primary endpoint of 12-week CTC count. PSA values post-treatment increased in 8 (89%) patients and remained unchanged in 1 (11%) patient. Conclusions: Single-agent anti-PSMA antibody J591 may lead to decline in CTC count, though the study did not meet its primary endpoint. A combination or maintenance approach might be preferable and is worthy of exploration.

Abstract CT122: Phase I dose-escalation study of prostate-specific membrane antigen (PSMA)-targeted alpha emitter 225Ac-J591 for progressive metastatic castration resistant prostate cancer (mCRPC)

Abstract Background: PSMA may be targeted by antibodies or small molecule ligands labeled with potent alpha emitters. Certain sites of PSMA expression such as salivary/lacrimal glands, kidneys, and small bowel are not accessible to antibodies such as J591. Following binding, immunoreactivity, and xenograft studies, we performed a first in human study of 225Ac-J591. Methods: Men with progressive mCRPC following at least 1 potent AR-targeted agent (e.g. abiraterone/enzalutamide) and taxane chemotherapy (or unfit/refuse chemo) without limit of # prior therapies (including radium-223 or prior PSMA therapy) provided adequate organ function were eligible. Baseline 68Ga-PSMA11 PET was performed, but not used for eligibility. Initially single-subject cohorts were treated until transition to 3+3 with dose level 5 (the dose level predicted by historical J591 dosimetry to have moderate toxicity) with a single infusion of 225Ac-J591 (13.3 KBq/kg with planned escalation up to 93.3 KBq/kg). Dose-limiting toxicity (DLT) was defined as attributable grade 4 hematologic toxicity or grade 3/4 non-heme toxicity. Imaging, genomic, patient-reported outcomes, and immune correlates are included. Results: In the dose-escalation phase, 22 men were treated on 7 dose levels; median age 72.5 (range 58-89), PSA 146.5 (4.8-7168.4); 82% with at least 2 prior potent AR pathway inhibitors, 64% chemo, 23% radium-223, 55% prior 177Lu-PSMA. 1 (4.5%) CALGB (Halabi) good prognostic risk, 10 (20.7%) intermediate, and 11 (50%) poor prognostic risk category. One of 6 subjects in cohort 6 (80 KBq/kg) had DLT (grade 4 anemia and platelets); no other subjects had any attributable grade >2 non-hematologic or grade >3 heme AE (including 0 of 6 at the highest dose level 93.3 KBq/Kg). Low grade temporary AE's include: 17 (77%) with fatigue, 11 (50%) pain flare, 11 (50%) nausea, 6 (27%) with Gr 1 xerostomia (5 of 6 with prior 177Lu-PSMA), 4 (18%) AST elevation. Despite the heavily pre-treated population including the majority with prior PSMA-TRT and no selection for PSMA expression, 14 (63.6%) with any PSA decline, 9 (40.9%) with >50% PSA decline. Interestingly, 15 (68%) had initial PSA increase followed by subsequent decline from peak (delayed effect). With follow up ongoing 2 with response > 1 year despite prior 177Lu-PSMA with correlative assays in process. Of 15 with paired baseline and 12-week CTC counts (CellSearch), 8 declined (45-100% decline), 4 remained undetectable, 3 increased. Conclusions: PSMA-targeted alpha-emitter 225Ac utilizing intact antibody J591 is tolerable with early evidence of clinical activity including long-term responders in a pre-treated population. Correlative studies are in progress and enrollment into a Simon 2-stage expansion cohort has been initiated. Clinicaltrials.gov NCT03276572 Citation Format: Scott T. Tagawa, Joseph Osborne, Charlene Thomas, Escarleth Fernandez, Muhammad J. Niaz, Shankar Vallabhajosula, Panagiotis Vlachostergios, Ana M. Molina, Cora Sternberg, Sharon Singh, Amie Patel, Angela Tan, John Babich, David M. Nanus, Karla Ballman, Neil H. Bander. Phase I dose-escalation study of prostate-specific membrane antigen (PSMA)-targeted alpha emitter 225Ac-J591 for progressive metastatic castration resistant prostate cancer (mCRPC) [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 CT122.

Review of Lutetium-177-labeled Anti-prostate-specific Membrane Antigen Monoclonal Antibody J591 for the Treatment of Metastatic Castration-resistant Prostate Cancer.

Prostate cancer is the most common non-cutaneous cancer in men in the United States and is the second most common cause of cancer deaths after lung cancer in men. Despite all advances in the field of prostate cancer imaging and treatment, currently, it is sub-optimally responsive to all available treatment options. Radioimmunotherapy with a monoclonal antibody (mAb), J591, has shown promising results in the treatment of prostate cancer. J591 is a deimmunized mAb that targets the extracellular domain of prostate-specific membrane antigen (PSMA), a surface-bound and internalizing glycoprotein that is upregulated in prostate cancer. Phase I/II clinical trials have shown accurate tumor targeting, biochemical and radiographic responses, and increased overall survival in patients with mCRPC with tolerable, predictable, and reversible myelotoxicity. Ongoing studies focus on improving the therapeutic index of radiolabeled J591. Herein, the literature on published clinical trials involving therapeutic J591 conjugated to b-emitter, lutetium-177 for mCRPC, is sequentially reviewed.

Phase I/II dose-escalation trial of fractionated dose 177Lu-J591 plus 177Lu-PSMA-617 for metastatic castration-resistant prostate cancer (mCRPC).

TPS339 Background: PC is a radiosensitive disease. PSMA is selectively overexpressed in advanced PC with upregulation by androgen receptor (AR) pathway dysregulation; limited expression exists in other organs. Prior studies of beta-emitting radiolabeled anti-PSMA antibody J591 demonstrated accurate targeting, efficacy with dose-response effect, and safety with predictable dose-limiting myelosuppression. Recent prospective trials have shown efficacy and safety of 177Lu-PSMA-617 in mCRPC. Studies demonstrate different binding sites of J591 and PSMA-617 and co-administration leads to non-competitive additive binding and delivery of payloads to PSMA+ cells. As the pharmacokinetics and biodistribution of the 2 agents is mostly non-overlapping (other than tumor) and given our prior dose-response data, we hypothesize that delivery of the combination will yield higher tumor delivery with less off-target toxicity (i.e. better responses without increased toxicity). Methods: Men with progressive mCRPC following at least 1 potent AR-targeted agent (e.g. abi/enza) and docetaxel (or unfit/refuse chemo) with metastatic disease on CT/MRI or bone scan without limit of # prior therapies (excluding bone-targeted beta-emitting radioisotopes) provided adequate organ function will be enrolled. Pre-treatment 88Ga-PSMA-11 will be performed, but results are not used for eligibility. Treatment includes fractionated 177Lu-J591 at a fixed moderate dose with escalating doses of 177Lu-PSMA-617 (7.4 – 18.5 GBq per cycle, fractionated D1 and D15) in a 3+3 dose-escalation study. Dose-limiting toxicity (DLT) is defined as attributable grade > 3 heme toxicity or grade > 2 non-heme toxicity. Following determination of recommended phase 2 dose (RP2D), the phase 2 portion will enroll in a 2-stage design. Primary endpoints: DLT and RP2D (ph 1) and PSA decline proportion (ph 2). Secondary endpoints include toxicity, radiographic response, PFS, rPFS, OS, CTC count changes. Correlatives include baseline/follow up PSMA imaging, whole body distribution of 177Lu, tissue and circulating genomic assessment, immunologic assessment, and patient reported outcomes (FACT-P and BPI-SF). Clinical trial information: NCT03545165.

Pilot study of dual imaging with 89Zr-IAB2M and 68Ga-PSMA-11 PET/CT prior to prostatectomy.

90 Background: Due to high expression of Prostate Specific Membrane Antigen (PSMA), it’s an excellent candidate for targeted molecular imaging. 89Zr-df-IAB2M is a J591 antibody derived minibody and 68Ga-PSMA-11 is a PSMA small molecule. Methods: Patients with clinically significant (defined as: ≥ 0.5 cm3 with Gleason pattern ≥ 4) localized prostate cancer (PCa) on conventional imaging modalities who planned to undergo surgery were imaged by PET/CT 1-3 hours after 5±2mCi of 68Ga-PSMA-11 injection and 89Zr-IAB2M PET 2-4 days after 10mg IAB2M labeled with 2.5 mCi of 89Zr injection. Image results were interpreted by a central reader without knowledge of the surgical pathology, mapped and the mapped findings later compared with surgical pathology map. Results: 9 patients having clinically localized PCa with median age 65 (46-79) and PSA 8.42 (1.6-12.2) were enrolled. All dominant PCa lesions, which had a median Gleason score 8 (6-9), were detected by both IAB2M and PSMA-11. The smallest of these dominant lesions was 8mm in size. Median SUVmax for dominant lesions was 3.3 (2.3-7.5) on IAB2M and 7.1 (1.6-23.2) on PSMA-11; median MR PIRADS was 4 (4-5). Of 9 Gleason 6 lesions identified on pathology, PSMA-11 detected 3 and IAB2M detected 2. Four extra-prostatic lesions in one patient were detected by both PET agents, 3 were confirmed by surgical pathology and 1 confirmed on bone scan 2 months post-op. A total of 8 lesions reported on scans were not reported by path: 3 were false positive on both scans, 3 were false positive on IAB2M scan only and 2 were false positive on PSMA-11 scan only. 5 Gleason 6 and 2 Gleason 3+4 lesions on pathology were missed by both PET agents and could be considered false negatives. Conclusions: In this small pilot series, performance of the both PET agents was very similar. An advantage of IAB2M is its hepatobiliary clearance rather than renal/urinary, which makes it potentially easier to visualize the PCa and/or pelvic disease; an advantage of PSMA-11 is the ability to image within 1 hour. Clinical trial information: NCT03675451.

Antitumor Activity of MEDI3726 (ADCT-401), a Pyrrolobenzodiazepine Antibody-Drug Conjugate Targeting PSMA, in Preclinical Models of Prostate Cancer.

Prostate-specific membrane antigen (PSMA) is a membrane-bound glutamate carboxypeptidase that is highly expressed in nearly all prostate cancers with the highest expression in metastatic castration-resistant prostate cancer (mCRPC). The prevalence of increased surface expression and constitutive internalization of PSMA make it an attractive target for an antibody-drug conjugate (ADC) approach to treating patients with mCRPC. MEDI3726 (previously known as ADCT-401) is an ADC consisting of an engineered version of the anti-PSMA antibody J591 site specifically conjugated to the pyrrolobenzodiazepine (PBD) dimer tesirine. MEDI3726 specifically binds the extracellular domain of PSMA and, once internalized, releases the PBD dimer to crosslink DNA and trigger cell death. In vitro, MEDI3726 demonstrated potent and specific cytotoxicity in a panel of PSMA-positive prostate cancer cell lines, consistent with internalization and DNA interstrand crosslinking. In vivo, MEDI3726 showed robust antitumor activity against the LNCaP and the castration-resistant CWR22Rv1 prostate cancer cell line xenografts. MEDI3726 also demonstrated durable antitumor activity in the PSMA-positive human prostate cancer patient-derived xenograft (PDX) LuCaP models. This activity correlated with increased phosphorylated Histone H2AX in tumor xenografts treated with MEDI3726. MEDI3726 is being evaluated in a phase I clinical trial as a treatment for patients with metastatic castrate-resistant prostate cancer (NCT02991911). Mol Cancer Ther; 17(10); 2176-86. ©2018 AACR.

Abstract A004: MEDI3726 (ADCT-401), a novel antibody-drug conjugate targeting PSMA, has potent in vivo antitumor activity in prostate cancer patient-derived xenograft models

Abstract Prostate specific membrane antigen (PSMA), a type II membrane glycoprotein, is highly expressed in nearly all prostate cancers, with the highest expression in metastatic castration-resistant prostate cancer (mCRPC). The prevalence, increased surface expression in prostate tumors, and constitutive internalization make PSMA an attractive target for an antibody-drug conjugate (ADC) approach in anticancer treatment of patients with mCRPC. MEDI3726 (previously known as ADCT-401) is an ADC consisting of an engineered version of anti-PSMA antibody J591, site-specifically conjugated with DNA cross-linking pyrrolobenzodiazepine (PBD) dimer SG3249 for targeting prostatic cancer cells. Using prostate cancer cell line models, we have previously shown that MEDI3726 specifically binds to the extracellular domain of PSMA and, once internalized, releases the PBD dimer to cross link DNA and achieve potent in vitro and in vivo cytotoxicity. Here we investigated the in vivo activity of MEDI3726 in a series of LuCaP prostate cancer patient-derived xenograft (PDX) models. The selected LuCaP models had varying PSMA expression and heterogeneous genetic and phenotypic backgrounds. In agreement with the earlier cell line xenograft data, dose-dependent antitumor activity was observed in PSMA-positive PDX models with durable tumor regressions in models with high PSMA expression. In the PSMA-negative LuCaP 35CR PDX model, MEDI3726 did not have significant antitumor activity, thus highlighting target-mediated in vivo activity. Increased phosphorylation of histone H2AX was observed in xenografts dosed with MEDI3726, confirming DNA damage induced by interstrand cross-linking PBD dimer as the mechanism of antitumor activity of MEDI3726. In summary, MEDI3726 demonstrated potent and specific in vivo antitumor activity, concurrent with DNA damage, in clinically relevant prostate cancer PDX models. MEDI3726 is being evaluated in phase 1 clinical trial as an anticancer treatment in patients with metastatic castrate-resistant prostate cancer (NCT02991911). Citation Format: Song Cho, Francesca Zammarchi, Noel R. Monks, Kapil Vashisht, Ravinder Tammali, Kevin Schifferli, Patrick Strout, Wanda King, Karma Dacosta, Ryan Fleming, David G. Williams, Karin Havenith, Mary Jane Masson Hinrichs, Simon Chivers, Nazzareno Dimasi, Phil W. Howard, John A. Hartley, Steve Coats, Ronald Herbst, Patrick H. van Berkel, David A. Tice. MEDI3726 (ADCT-401), a novel antibody-drug conjugate targeting PSMA, has potent in vivo antitumor activity in prostate cancer patient-derived xenograft models [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A004.

Phase I dose-escalation study of 225Ac-J591 for progressive metastatic castration resistant prostate cancer (mCRPC).

TPS399 Background: PC is a radiosensitive disease. PSMA is overexpressed in advanced PC with upregulation by androgen receptor (AR) pathway dysregulation; limited expression exists in other organs. A series of sequential studies of beta-emitting radiolabeled anti-PSMA monoclonal antibody (mAb) J591 have demonstrated accurate targeting, efficacy with dose-response effect, and safety with predictable, reversible dose-limiting myelosuppression. Alpha emitters are significantly more potent with a shorter range than beta emitters. Though there is no direct tumor-targeting, the bone-targeting alpha emitter Ra223 is approved. Anecdotal reports of PSMA small molecule targeted alpha emitters have hinted at efficacy but are limited by xerostomia and in mouse models may lead to long-term renal damage. Intact mAb J591 has comparatively no to minimal distribution in salivary glands and kidneys. Preclinical studies demonstrated purity, immunoreactivity, and stability with efficacy in a xenograft model [AACR 2017]. Methods: Men with progressive mCRPC following at least 1 potent AR-targeted agent (e.g. abi/enza) and docetaxel (or unfit/refuse chemo) without limit of # prior therapies (excluding beta-emitting bone-targeted radioisotopes) provided adequate organ function will undergo imaging with 68Ga-PSMA-11 PET/CT followed by a single dose of 225Ac-J591. Single-subject cohorts will be enrolled until grade > 1 attributable toxicity, then transition to 3+3 design. Cohort 1 = 13.3 KBq/kg with planned escalation up to 93.3 KBq/kg of 225Ac with fixed 20 mg J591. Dose-limiting toxicity (DLT) is defined as attributable grade 4 heme toxicity or grade 3/4 non-heme toxicity. Planned cohort expansion will occur at recommended phase 2 dose (RP2D) in a 2-stage design. The primary endpoint is determination of DLT and RP2D. Secondary endpoints include toxicity, PSA decline rate, RECIST response, PFS, rPFS, OS, and patient reported outcomes (FACT-P and BPI-SF). Correlatives include baseline/follow up PSMA imaging, CTC count (CellSearch) changes, tissue and circulating genomic assessment, and immune studies. Enrollment began in October, 2017. Clinical trial information: NCT03276572.

Abstract 5201: PSMA-targeted alpha radioimmunotherapy for prostate cancer with 225Ac-J591

Abstract Background: Prostate cancer (PC) is a radiosensitive disease. In addition to external beam and brachytherapy, systemic bone-seeking radioisotopes have been utilized clinically for many years, with radium-223 leading to overall survival benefits. However, the available beta and alpha emitting particles in the clinic target tumors / stroma in bone only. Biologically targeted radiopharmaceutical treatment has shown promising results in our earlier multiple clinical trials using beta-emitting anti-PSMA therapy with Leutetium-177 radiolabeled anti-PSMA monoclonal antibody (mAb) J591, with accurate targeting and tumor response, but treatment is limited by myelosuppression. Both beta and alpha-small molecule PSMA ligand directed therapy has been utilized anecdotally outside of the US, but has not been systemically studied. Based upon their biodistribution, targeting of salivary glands and potentially proximal renal tubules may pose longer term toxicity issues. mAb-based targeted alpha-particle delivery may be advantageous with less expected myelosuppression than seen with β-emitters. Experimental procedures: To optimize labeling of humanized DOTA-J591 mAb, actinium-225 (225Ac) nitrate was obtained from Oak Ridge National Laboratories and was dissolved in HCl solution and then mixed with trimethyl ammonium acetate and L-ascorbic acid. This preparation was incubated with DOTA-J591. The labeling efficiency (LE) was determined by ITLC-SG. If the LE is >60%, 225Ac-DOTA-J591 mAb was purified by gel filtration and sterilized by membrane filtration. In a pilot pre-clinical study, we studied the toxicity in non-tumor bearing BALB/c mice. Results: The radiochemical purity is >95% and the immunoreactivity was >80%. Twenty-five BALB/c mice received from 2.1 to 6.3 KBq of 225Ac-J591. Treatment was well tolerated, with all mice alive and healthy at Day 26. Conclusions: A stable alpha emitting-antibody complex has been produced with 225Ac-J591. Initial mouse safety experiments have been successful. A RIT dose escalation study of PSMA-targeted alpha emitter in a LNCaP xenograft model is underway. Citation Format: Jaspreet S. Batra, He Liu, Sae Kim, Vicente N. Navarro, Shankar Vallabhajosula, Scott T. Tagawa, Neil H. Bander. PSMA-targeted alpha radioimmunotherapy for prostate cancer with 225Ac-J591 [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 5201. doi:10.1158/1538-7445.AM2017-5201

Abstract 3799: Technical validation of a micro-flow cytometry platform for prostate cancer biomarker discovery

Abstract The analysis of extracellular vesicles (EVs) in biofluids is an expanding area of biomarker discovery. To be effective, a diagnostic test must differentiate EVs originating from cancer cells versus non-cancerous origins, and also define biomarkers enriched in cancer EVs. Our team has developed a micro-flow cytometry platform using the Apogee A50 to analyze prostate cancer (PCa) EVs in different biofluids. While flow cytometry is powerful tool for quantitative and qualitative evaluation of cellular biomarkers, direct translation of most applications to EVs is challenging. For example, EVs in biofluids typically range in size from 50-1000nm, and are present as low refractive index, polydisperse populations. We have optimized 30 assay parameters for the identification of prostate specific EVs so that disease specific biomarkers may be evaluated. To illustrate the sensitivity of the system, conditioned media from palmitoylated-GFP-LNCaP cells was used as a positive signal sample. When spiked into increasing concentrations of unstained plasma EVs, the GFP spike was detected linearly, even when present as low as 0.03% of the total EV population. Furthermore, the rate of positive signal/uL sample was quantitative after 15-30 seconds of sample analysis. The platform also permits high throughput analysis using a 96 well plate autosampler. To show sample reproducibility, we analyzed both low and high positive-signal samples over an 8hr sampling window. Low positive samples (120events/uL; 0.1% of total EVs) had a 4% CV over 96 samples whereas the high positive signal samples (2300events/uL, 1.0% of total EVs) had a 2.2% CV. Similar CVs have been obtained with CD9 and CD63. Analysis of sample aliquots over multiple days shows less than 6% CV. Conventional flow cytometers acquire tens of thousands of events while the Apogee system is able to collect millions of events with several parameters/event, in a high throughput manner. This generates a significant amount of data for which we have designed a unique machine learning approach to detect specific EV populations within the whole sample. In this manner, we utilize all data from the sample and not simply a subjectively defined area of interest. Using fresh samples from the clinic we have standardized a 2hr arm-freezer protocol for the collection and preservation of serum and plasma. Comparison studies in LNCaps and PC3 cells of 5 antibodies against PSMA, indicated the J591 anti-PSMA monoclonal antibody as a selective marker for prostate cancer EVs. Positive signal for this antibody was linearly titrated in both a LNCaP EV system and plasma EVs from metastatic prostate cancer patients. Specificity was shown by saturating a plasma system with unconjugated J591 and then reacting with PE-conjugated J591. Using the A50 micro-flow cytometry platform, the J591 antibody to identify prostate specific EVs, and a unique machine learning algorithm, we identified a multiple biomarker, diagnostic signature for PCa. Citation Format: Desmond Pink, Robert Paproski, Deborah Sosnowski, Lian Willetts, Eric Hyndman, John D. Lewis. Technical validation of a micro-flow cytometry platform for prostate cancer biomarker discovery [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 3799. doi:10.1158/1538-7445.AM2017-3799

Phase I dose-escalation study of fractionated-dose 177Lu-PSMA-617 for progressive metastatic castration resistant prostate cancer (mCRPC).

TPS5093 Background: PC is a radiosensitive disease. PSMA is selectively overexpressed in advanced PC with upregulation by androgen receptor (AR) pathway dysregulation; limited expression exists in other organs. A series of sequential studies of radiolabeled anti-PSMA antibody J591 revealed 1) targeting and safety [Bander 2003]; 2) safety and prelim efficacy [Milowsky 2004, Bander 2005]; 3) efficacy and initial dose-response [Tagawa 2013]; 4) dose-fractionation allows higher doses, ability to combine with docetaxel, confirmation of dose-response (PSA and overall survival) [ASCO 2010, 2014, 2016]; 5) predictable, reversible myelosuppression is dose-limiting [Tagawa 2013]. Small molecule PSMA inhibitor ligands can be successfully radiolabeled and are widely used for imaging and treatment in Europe. 177Lu-PSMA-617 is the most commonly used, but experience is mostly anecdotal/retrospective and no formal dose-escalation studies have been performed. Methods: Men with progressive mCRPC following at least 1 potent AR-targeted agent (e.g. abi/enza) and docetaxel (or unfit/refuse chemo) without limit of # prior therapies provided adequate organ function will undergo imaging with 68Ga-PSMA-HBED-CC PET/CT followed by escalating fractionated doses of 177Lu-PSMA-617. Cohort 1 = 3.7 GBq x2 two weeks apart up to 11.1 GBq x2 in a 3+3 dose-escalation study. Dose-limiting toxicity (DLT) is defined as attributable grade 4 heme toxicity or grade 3/4 non-heme toxicity. Planned cohort expansion will occur at recommended phase 2 dose (RP2D) in a 2-stage design. The primary endpoint is determination of DLT and RP2D. Secondary endpoints include toxicity, PSA decline rate, RECIST response, PFS, rPFS, OS. Correlatives include baseline/follow up PSMA imaging, whole body distribution of 177Lu-PSMA-617, CTC count (CellSearch) changes, tissue and circulating genomic assessment of DNA repair pathways, patient reported outcomes (FACT-P and BPI-SF). Clinical trial information: NCT03042468.

Abstract 3971: Non-invasive assessment of prostate-specific membrane antigen (PSMA) expression as a prognostic marker in men with metastatic castration-resistant prostate cancer (mCRPC)

Abstract Background: PSMA is nearly universally expressed by PC, has a correlation with androgen receptor (AR) pathway dysregulation, and some studies have pointed towards PSMA expression as a prognostic marker. Non-invasive measurement of PSMA expression may be biomarker of AR activity in animal models.[Evans et al, PNAS 2011] Anti-PSMA monoclonal antibody J591 has been utilized both for imaging as well as therapeutics and we have published semi-quantitative scoring in the setting of therapeutic clinical trials. Methods: Patients with mCRPC underwent planar gamma camera imaging following radiolabeled (RL) J591 injection (177Lu- or 111In-J591). RL-J591 images were semi-quantitatively scored using a 5-point visual score (VS) system [Tagawa et al, Clin Cancer Res 2013]. Overall survival (OS) was calculated from the date of imaging to death or last follow up. As several life-prolonging treatments (docetaxel, sipuleucel-T, cabazitaxel, abiraterone (Abi), enzalutamide (Enza), radium-223) became available during the study time period, treatment following RL-J591 imaging was recorded and used in multivariate analysis. Results: Between 2000 and 2015, 165 patients with mCRPC were imaged following RL-J591 and semi-quantitatively scored for PSMA expression by VS. Baseline demographics included median age of 71.3 years (44.5-85.9); bone metastases present in 86.7%, 52.7% with LN mets, 17% with lung mets, 7.9% with liver mets, 1.8% other mets. Fifty three (32.2%) had low PSMA expression by imaging (VS 0-1) and 112 (67.8%) had high PSMA expression (VS 2-5). Post RL-J591 imaging, 41.2% (68/165) patients received life prolonging therapy, including taxane chemotherapy (33.9%), Abi/Enza (14.5%), sipuleucel-T (1.2%), or radium-223 (1.2%). At last follow up 12.7% (21/165) patients were alive. Median OS was 22.6 months in patients with low PSMA expression and 16.6 mo with high PSMA expression (P = 0.07). Those with subsequent receipt of life prolonging therapy had OS of 23.6 months vs 14.3 mo (P = 0.002). On multivariable analysis controlling for life prolonging therapy, higher PSMA expression by imaging was a significant predictor for poorer OS (adjusted hazard ratio = 1.60; 95% CI = 1.10, 2.31; P = 0.01). Conclusion: Non-invasive measurement of PSMA expression may be evaluated on a semi-quantitative basis via planar imaging. The level of PSMA expression is inversely associated with survival when controlling for receipt of known life-prolonging therapy. With the introduction of newer quantitative molecular imaging (i.e. PET), quantitative PSMA imaging may prove to be a useful prognostic tool. Citation Format: Kavya Pinto-Chengot, Yuliya Jhanwar, Jaspreet Batra, Beerinder Karir, Shankar Vallabhajosula, Paul Christos, Ana Molina, Himisha Beltran, David M. Nanus, Stanley J. Goldsmith, Neil H. Bander, Scott T. Tagawa. Non-invasive assessment of prostate-specific membrane antigen (PSMA) expression as a prognostic marker in men with metastatic castration-resistant prostate cancer (mCRPC). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3971.

Targeting of radiolabeled J591 antibody to PSMA-expressing tumors: optimization of imaging and therapy based on non-linear compartmental modeling.

BackgroundWe applied a non-linear immunokinetic model to quantitatively compare absolute antibody uptake and turnover in subcutaneous LNCaP human prostate cancer (PCa) xenografts of two radiolabeled forms of the humanized anti-prostate-specific membrane antigen (PSMA) monoclonal antibody J591 (124I-J591 and 89Zr-J591). Using the model, we examined the impact of dose on the tumor and plasma positron emission tomography (PET)-derived time-activity curves. We also sought to predict the optimal targeting index (ratio of integrated-tumor-to-integrated-plasma activity concentrations) for radioimmunotherapy.MethodsThe equilibrium rates of antibody internalization and turnover in the tumors were derived from PET images up to 96 h post-injection using compartmental modeling with a non-linear transfer rate. In addition, we serially imaged groups of LNCaP tumor-bearing mice injected with 89Zr-J591 antibody doses ranging from antigen subsaturating to saturating to examine the suitability of using a non-linear approach and derived the time-integrated concentration (in μM∙hours) of administered tracer in tumor as a function of the administered dose of antibody.ResultsThe comparison of 124I-J591 and 89Zr-J591 yielded similar model-derived values of the total antigen concentration and internalization rate. The association equilibrium constant (ka) was twofold higher for 124I, but there was a ~tenfold greater tumoral efflux rate of 124I from tumor compared to that of 89Zr. Plots of surface-bound and internalized radiotracers indicate similar behavior up to 24 h p.i. for both 124I-J591 and 89Zr-J591, with the effect of differential clearance rates becoming apparent after about 35 h p.i. Estimates of J591/PSMA complex turnover were 3.9–90.5 × 1012 (for doses from 60 to 240 μg) molecules per hour per gram of tumor (20 % of receptors internalized per hour).ConclusionsUsing quantitative compartmental model methods, surface binding and internalization rates were shown to be similar for both 124I-J591 and 89Zr-J591 forms, as expected. The large difference in clearance rates of the radioactivity from the tumor is likely due to differential trapping of residualizing zirconium versus non-residualizing iodine. Our non-linear model was found to be superior to a conventional linear model. This finding and the calculated activity persistence time in tumor have important implications for radioimmunotherapy and other antibody-based therapies in patients.

Abstract CT306: Radiolabeled anti-PSMA antibody J591 immunotherapy is associated with favorable circulating tumor cell (CTC) count control in men with castration-resistant prostate cancer

Abstract Background: J591 is a monoclonal antibody that selectively binds to prostate-specific membrane antigen (PSMA). Prospective phase I and II trials with therapeutic radiolabeled 177Lu-J591 have shown favorable PSA decline rates. Additionally, our group has previously reported that non-invasive evaluation of PSMA expression by imaging with planar gamma camera imaging is associated with response and survival. Initial anecdotal experience showed declines in CTC counts which led to prospective study. Methods: Three clinical trials of 177Lu-J591 in men with metastatic CRPC have included prospective evaluation of CTC counts (CellSearch methodology) prior to and following therapy (single-dose 177Lu-J591, fractionated-dose 177Lu-J591, and fractionated-dose 177Lu-J591 + docetaxel). Based upon prior experience of J591-based therapy without an effector molecule as immunotherapy, and an anecdotal report of CTC count decline following a low (imaging) dose of J591 without an effector molecule, a cohort of patients who received J591 without 177Lu labeling was also retrospectively analyzed. Results: 44 pts unselected for PSMA expression received 177Lu-J591 therapy had prospectively measured CTC counts. 39 of 44 pts (88.6%) had imaging evidence of PSMA expression. 57% of pts had baseline unfavorable (≥ 5 cells/7.5 mL) CTC counts. Of overall evaluable pts, 72.4% had decline in CTC counts (7.5 to 100% decline), with 50% of those with unfavorable baseline counts converting to favorable counts at 4-6 weeks. 17.1% with undetectable CTC counts remained undetectable at follow up. 7 pts with mCRPC and elevated CTC counts who received low-dose (naked J591) prior to imaging were also identified, and 4 (57%) of these pts demonstrated CTC count decline. Conclusion: In addition to previously reported PSA declines, treatment with 177Lu-J591 is associated with favorable CTC count decline in men with metastatic CRPC. Infusion with unlabeled J591 may also lead to CTC count clearance. Based on this finding, we are planning a prospective clinical trial with unlabeled (naked) J591 in men with metastatic CRPC and unfavorable CTC counts. Citation Format: Pravin Date, Beerinder S. Karir, Jaspreet S. Batra, Yuliya Jhanwar, Himisha Beltran, David M. Nanus, Neil H. Bander, Scott T. Tagawa. Radiolabeled anti-PSMA antibody J591 immunotherapy is associated with favorable circulating tumor cell (CTC) count control in men with castration-resistant prostate cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr CT306. doi:10.1158/1538-7445.AM2015-CT306

Indium 111-labeled J591 anti-PSMA antibody for vascular targeted imaging in progressive solid tumors.

BackgroundJ591 is a monoclonal antibody that targets the external domain of the prostate-specific membrane antigen (PSMA). Besides prostate cancer cells, it also targets the neovasculature of non-prostate solid tumors. We provide an analysis of the antibody mass-dose dependency of lesion uptake and normal tissue retention, together with an assessment of lesion detectability using 111In-J591 imaging, compared with conventional imaging in patients with a variety of solid tumors.MethodsTwenty patients in six cohorts received fixed amounts (5, 10, 20, 40, 60, and 100 mg) of J591 in a phase I trial. A maximum of four administrations per patient was given, with each administration separated by 3 weeks. All antibody administrations included 370 MBq (10 mCi) of 111In labeled to 2 mg of J591 via the chelating agent DOTA. Three whole body (WB) gamma camera scans with at least one SPECT scan, along with multiple WB count-rate measurements and blood samples, were obtained for all patients. The effect of escalating antibody mass on lesion uptake and normal tissue retention was evaluated using lesion, liver, serum, and WB residence times and ratios thereof for each treatment cycle. Lesion detectability using 111In-J591 imaging was compared to the standard imaging on a lesion-by-lesion basis.ResultsA total of 170 lesions in 20 patients were detected by standard or 111In-J591 imaging. 111In-J591 targeted both skeletal and soft tissue diseases in all tumor types. 111In-J591 imaging identified 74% (20/27) of skeletal lesions, 53% (18/34) of nodes, and 64% (70/109) of other soft tissue/organ lesions. There was increasing 111In-J591 uptake in lesions with increasing antibody mass-dose, coupled with decreasing retention in the liver for increments up to 20 mg, and no significant change at higher antibody mass.ConclusionsRadiolabeled J591 antibody has potential as a targeting agent for solid tumor vasculature and lesion detection. Bone and soft tissue lesions arising from tumors of diverse origin were targeted by the anti-PSMA antibody J591. For the detection of lesions in these tumors by J591 antibody scans, an antibody mass of 20 mg is adequate. The optimal time of imaging is 5 to 7 days post-injection.

Pathologic correlation of 89Zr-Df-IAB2M antiprostate-specific membrane antigen (PSMA) minibody in patients with metastatic prostate cancer.

220 Background: IAB2M is a novel anti-PSMA minibody (Mb) based on the humanized J591 antibody that targets the extracellular domain of PSMA. The Mb has been engineered to remove Fc-Rn interactions and reduce the molecular weight relative to J591. The result is faster blood pool clearance, for a more favorable imaging schedule, while retaining bivalent targeting of PSMA. We have previously reported on the pharmacokinetics, dosimetry, and lesion uptake of IAB2M(Pandit-Taskar et al, WMIS 2014). Here we report the correlation between imaging and pathology of biopsies of PSMA avid lesions. Methods: Following standard imaging (SI) of CT/MRI, bone scintigraphy (BS), and FDG PET, 5 mCi of 89Zr-Df-IAB2M was administered intravenously. Escalating minibody doses of 10, 20, and 50 mg were delivered, with cohort expansion at 10 and 20 mg. Whole body PET/CT scans were serially performed at various time points:1-2 h, 24 h, 48 h, and 72-144 h post injection. Metastases identified on PET were confirmed, where possible, with bx’s in the following preference: concordant IAB2M and FDG positivity, IAB2M, and FDG mismatch, or a mismatch between SI and any PET. Results: 24 patients (pts) with metastatic prostate cancer were scanned (11 at 10, 7 at 20, and 6 at 50 mg Mb doses). A total of 15 bxs (7 soft tissue, 8 bone) were performed on 14 pts; the histopathologic correlation is summarized in the table below. 13 lesions were identified by IAB2M, 11 by FDG, and 14 by SI. Of these, 12/13 (92.3%), 10/11 (90.9%), 12/14 (85.7%) were biopsy positive for cancer, respectively. 2 bone lesions were not identified by IAB2M, evident on other imaging modalities, and were both neg by bx. Overall bx concordance (pos/pos, neg/neg) with imaging was: IAB2M 14/15 (93%) vs. BS/CT 13/15 (86%) vs. FDG 12/15 (80%). Conclusions: An ongoing analysis of IAB2M imaging showed a high concordance with pathologic findings in patients with metastatic prostate cancer. Clinical trial information: NCT01923727. [Table: see text]