Phase 1 study of P-PSMA-101 CAR-T cells in patients with metastatic castration-resistant prostate cancer (mCRPC)

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Background: P-PSMA-101 is an autologous CAR-T therapy targeting PSMA, with a high percentage of stem cell memory T cells (TSCM) associated with efficacy, safety, and bone homing (particularly relevant to prostate cancer). It is manufactured using a novel non-viral transposon system (piggyBac) that creates high TSCM products. Genes are inserted encoding a PSMA-targeted Centyrin CAR, iCasp9-based safety switch, and DHFR to purify CAR-T cells. P-PSMA-101 completely eliminated tumors in intractable murine models of prostate cancer, providing rationale for this phase 1 trial (NCT04249947). Methods: Patients with mCRPC treated with or not eligible for a CYP17 inhibitor or second-generation antiandrogen, and a taxane were enrolled. P-PSMA-101 was manufactured from apheresed T cells and administered IV following a standard 3-day cy/flu lymphodepletion regimen. Dose escalation from 0.25-15 x 106 cells/kg is planned. Results: As of September 30, 2021, P-PSMA-101 had been administered to 10 heavily pretreated patients (median 7 prior regimens; range 3-15). Single infusions of 0.25 (n=5) to 0.75 (n=5) x 106 cells/kg have been assessed, with dose escalation continuing. P-PSMA-101 cells were shown to expand in blood via qPCR assay, peaking 2-3 weeks after infusion, consistent with the high percentage of TSCM. Significant antitumor responses were seen in this preliminary data set. Declines in PSA were seen in 7 patients (>50% in 3 and >99% in 1). Of 4 patients who had pre- and post-treatment FDG and PSMA-PET imaging, 3 demonstrated marked to complete resolution of abnormal uptake at known metastatic disease sites, with concordance in bone and CT scans, and/or circulating tumor cells (CTC). In 1 case, post-treatment tumor biopsy demonstrated infiltration by P-PSMA-101 CAR-T cells and elimination of tumor cells (pathologic complete response). Safety was consistent with expectations for a CAR-T product. CRS was seen in 60% (10% Gr ≥3) of patients. DLT was seen in 1 patient with macrophage activation syndrome/uveitis, and was the only Gr ≥3 CRS event. Immune effector cell-associated neurotoxicity syndrome (ICANS) has not occurred. CRS marker elevations were modest (max IL-6: 642.6 pg/mL). The most common AEs were cytopenias, infections, and constitutional symptoms (Gr ≥3 60%, 10%, and 0%), as expected with lymphodepletion. Treatable related ocular AEs were noted in 3 patients. Conclusions: These results parallel preclinical findings that P-PSMA-101 can produce marked efficacy in mCRPC, and very low doses are highly efficacious, consistent with unique product attributes such as the TSCM phenotype and bone tropism. This is the first report demonstrating profound antitumor effects of a novel PSMA-directed CAR-T-cell platform with concordant biochemical, radiographic, and pathologic parameters, demonstrating that therapeutic benefit of unarmored CAR-T cells in a major solid tumor is possible. Clinical trial information: NCT04249947.

123 P-MUC1C-ALLO1: A fully allogeneic stem cell memory T cell (TSCM) CAR-T therapy with broad potential in solid tumor

Background

While CAR-T have demonstrated potent activity against hematologic tumors, less success has been seen with solid tumors. Here we report generation of TSCM-enriched allogeneic MUC1-C-specific CAR T cells, P-MUC1C-ALLO1, with potential for a broad range of solid tumors. The proliferative capacity and metabolic profile of TSCM CAR-T are well-suited to activity in the solid tumor setting. MUC1 is comprised of an N-terminal subunit (MUC1-N) tethered to a C-terminal subunit (MUC1-C), forming a stable complex on the cell surface. During tumorigenesis, MUC1 becomes both overexpressed and hypo-glycosylated on many carcinomas. Furthermore, MUC1 undergoes proteolytic cleavage in the tumor microenvironment, leaving behind a proteolytic ‘stump’ of MUC1-C that is over-represented in cancer, making it an attractive therapeutic target.

Methods

P-MUC1C-ALLO1 is manufactured using the piggyBac® DNA Delivery System for CAR insertion and the Cas-CLOVER™ Gene Editing System to knockout both the TCR and MHC class I proteins. The addition of a selectable marker within the transposon allows for selection of a fully CAR-positive population while any residual TCR-positive cells are removed at the end of production to prevent TCR-mediated GvHD. Lastly, inclusion of a proprietary ‘booster molecule’ in our allogeneic process further improves cell expansion, along with phenotype and function, and enables the production of up to hundreds of patient doses from a single manufacturing run.

Results

Significant doses of P-MUC1C-ALLO1 products made from multiple healthy donors were achieved and comprised of an exceptionally high-percentage of desirable TSCM cells. Preclinical evaluation of these products showed potent tumor killing and cytokine secretion against MUC1-C-positive breast and ovarian tumor cell lines. P-MUC1C-ALLO1 demonstrates potent cytotoxicity against tumor cells, and minimal killing of normal MUC1-C-positive human primary cells. In a triple negative breast cancer xenograft model, MUC1C CAR-T eliminated established MDA-MB-468 tumor cells, mounted robust T cell expansion in peripheral blood and maintained a favorable TSCM percentage over time. Likewise, in an orthotopic ovarian cancer xenograft model, intraperitoneally administered MUC1C CAR-T eliminated established OVCAR3 cells to levels below the limit of detection. All together, these data demonstrated the efficacy of the MUC1C CAR-T cells and the robustness of the allogeneic platform.

Conclusions

P-MUC1C-ALLO1 is an allogeneic TSCM CAR-T therapy that has a potential to treat multiple MUC1-expressing indications. P-MUC1C-ALLO1 displayed specificity for tumor vs. normal cells as well as in vivo efficacy against xenograft models of breast and ovarian cancer. This allogeneic cell therapy is advancing rapidly towards the clinic.

147 Memory phenotype in allogeneic anti-BCMA CAR-T cell therapy (P-BCMA-ALLO1) correlates with in vivo tumor control

Background

The emergence of CAR-T cell therapy has transformed the treatment of refractory/relapsed multiple myeloma (MM). Yet, autologous CAR-T cells suffer from many manufacturing challenges including mainly consistency, toxicity, and cost. To address these issues, we engineered a fully allogeneic anti-BCMA CAR-T cell candidate for MM from healthy donors (P-BCMA-ALLO1). Herein, we demonstrate that this therapy maintains a stem cell memory T cell (TSCM) phenotype through editing which correlates with in vivo antitumor efficacy.

Methods

Using Poseida’s non-viral piggyBac® (PB) DNA Delivery System in combination with the high-fidelity Cas-CLOVER™ (CC) Site-Specific Gene Editing System and a proprietary ‘booster molecule’, we generated P-BCMA-ALLO1 from healthy donor T cells. We used CC to eliminate surface expression of both the TCR and MHC class I to make fully allogeneic CAR-T cells. In addition to the CAR molecule, PB enables the delivery of a selectable marker allowing the generation of a final cell product that is >95% CAR-positive. The inclusion of the ‘booster molecule’ in the manufacturing process improves the expansion of gene-edited cells without compromising memory phenotype or function. This process can produce up to hundreds of patient doses from a single manufacturing run which significantly reduces manufacturing cost per dose. We characterized the memory phenotype of P-BCMA-ALLO1 by assessing the mRNA and protein expression profiles of rested and activated CAR-T cells by flow cytometry and Nanostring analysis. We also assessed the antitumor capabilities of these cells using cytotoxicity assays and performed serial in vitro restimulation to assess the ability of P-BCMA-ALLO1 to undergo multiple rounds of activation and expansion. We then evaluated the relationship of these characteristics with in vivo efficacy, as defined by control of tumor in an immunodeficient RPMI-8226 subcutaneous murine tumor model.

Results

P-BCMA-ALLO1 is comprised of a high frequency of TSCM. It has potent in vivo antitumor activity, which is comparable to non-edited autologous anti-BCMA CAR-T cell therapy. Expression of memory markers at both mRNA and protein levels across individual lots significantly correlates with in vivo tumor control. Conversely, suboptimal research products with worse in vivo outcomes expressed an exhausted gene expression profile. Moreover, CAR-T products that are more effective in vivo are also more viable, cytotoxic, and proliferative following multiple rounds of restimulation in vitro.

Conclusions

P-BCMA-ALLO1 is a highly potent and safe allogeneic anti-BCMA CAR with a manufacturing process that consistently maintains a TSCM phenotype, which correlates with antitumor efficacy. P-BCMA-ALLO1 is advancing rapidly towards the clinic (NCT04960579).

A novel non-agonist c-Met antibody drug conjugate with superior potency over a c-Met tyrosine kinase inhibitor in c-Met amplified and non-amplified cancers

c-Met is a well-characterized oncogene that is associated with poor prognosis in many solid tumor types. While responses to c-Met inhibitors have been observed in clinical trials, activity appears to be limited to those with MET gene amplifications or mutations. We developed a c-Met targeted antibody-drug conjugate (ADC) with preclinical activity in the absence of MET gene amplification or mutation, and activity even in the context of moderate protein expression. The ADC utilized a high-affinity c-Met antibody (P3D12), that induced c-Met degradation with minimal activation of c-Met signaling, or mitogenic effect. P3D12 was conjugated to the tubulin inhibitor toxin MMAF via a cleavable linker (vc-MMAF). P3D12-vc-MMAF demonstrated potent in vitro activity in c-Met protein-expressing cell lines regardless of MET gene amplification or mutation status, and retained activity in cell lines with medium-low c-Met protein expression. In contrast, the c-Met tyrosine kinase inhibitor (TKI) PHA-665752 slowed tumor cell growth in vitro only in the context of MET gene amplification or very high protein expression. This differential activity was even more marked in vivo. P3D12-vc-MMAF demonstrated robust inhibition of tumor growth in the MET gene amplified MKN-45 xenograft model, and similar results in H1975, which expresses moderate levels of wild type c-Met without genomic amplification. By comparison, the c-Met TKI, PHA-665752, demonstrated modest tumor growth inhibition in MKN-45, and no inhibition at all in H1975. Taken together, these data suggest that P3D12-vc-MMAF may have a superior clinical profile in treating c-Met positive malignancies in contrast to c-Met pathway inhibitors.

TR1801-ADC: a highly potent cMet antibody-drug conjugate with high activity in patient-derived xenograft models of solid tumors

cMet is a well‐characterized oncogene that is the target of many drugs including small molecule and biologic pathway inhibitors, and, more recently, antibody–drug conjugates (ADCs). However, the clinical benefit from cMet‐targeted therapy has been limited. We developed a novel cMet‐targeted ‘third‐generation’ ADC, TR1801‐ADC, that was optimized at different levels including specificity, stability, toxin–linker, conjugation site, and in vivo efficacy. Our nonagonistic cMet antibody was site‐specifically conjugated to the pyrrolobenzodiazepine (PBD) toxin–linker tesirine and has picomolar activity in cancer cell lines derived from different solid tumors including lung, colorectal, and gastric cancers. The potency of our cMet ADC is independent of MET gene copy number, and its antitumor activity was high not only in high cMet‐expressing cell lines but also in medium‐to‐low cMet cell lines (40 000–90 000 cMet/cell) in which a cMet ADC with tubulin inhibitor payload was considerably less potent. In vivo xenografts with low–medium cMet expression were also very responsive to TR1801‐ADC at a single dose, while a cMet ADC using a tubulin inhibitor showed a substantially reduced efficacy. Furthermore, TR1801‐ADC had excellent efficacy with significant antitumor activity in 90% of tested patient‐derived xenograft models of gastric, colorectal, and head and neck cancers: 7 of 10 gastric models, 4 of 10 colorectal cancer models, and 3 of 10 head and neck cancer models showed complete tumor regression after a single‐dose administration. Altogether, TR1801‐ADC is a new generation cMet ADC with best‐in‐class preclinical efficacy and good tolerability in rats.

Abstract 225: TR1801-ADC, an optimized anti cMet PBD ADC with high efficacy in solid tumors of the GI tract and head & neck cancer

cMet is a well-characterized oncogene that is the target of many drugs including small molecule and biologic pathway inhibitors, and more recently, antibody drug conjugates (ADCs). However, clinical benefit from c-Met targeted therapy has been limited up to this point. We developed an optimized c-Met targeted antibody drug conjugate TR1801-ADC that utilized optimization at all steps including specificity, stability, internalization, toxin linkers, conjugation site, PK, and in vivo efficacy. The results were a highly potent c-Met ADC that was superior in comparison to a conventional MMAE (monomethylauristatin E) cMet ADC in potency, efficacy and duration of response. TR1801-ADC is site-specifically conjugated to a PBD-toxin linker and has low picomolar activity in multiple cancer cell lines derived from different solid tumors including lung cancer, colorectal cancer and gastric cancer. The potency of our cMet ADC is independent of MET gene copy number and its activity was high not only in high cMet cell lines but also in medium to low cell lines (100,000 to 40,000 cMet/cell). We identified potential Phase 1 cancer indications based on IHC and cMet H-score from tissue micro arrays (TMAs). 3 cancer indications (gastric, colorectal and head & neck cancer) were chosen based on high abundance of high cMet expression (>20% abundance of H-Score higher than 150) and were used to assess the efficacy of our cMet ADC in PDX models. For each indication 10 PDX (patient-derived xenograft) models were chosen based on their cMet expression. TR1801-ADC was administered in all models as single i.v. dose at 1, 0.5, 0.25 and 0.125 mg/kg. Control ADC was administered at 1mg/kg. Tumor growth inhibition varying from >100% to around 40% was seen in all gastric cancer PDX models with single dose injections of 1 mg/kg and 0.5 mg/kg. Complete tumor regression was seen in 70% of the 1 mg/kg group, in 50% of the 0.5 mg/kg group and in 40% of the 0.25 mg/kg group. TR1801-ADC produced statistically significant growth inhibition in comparison to untreated controls in 9 colorectal cancer PDX models. Complete tumor regression was observed in 40% (4/10) PDX models when treated with 1 mg/kg TR1801-ADC. The further 50% showed partial regression (5/10 models) and one showed no significant tumor response. The head & neck cancer PDX models were in general least sensitive to TR1801-ADC. TR1801-ADC produced statistically significant growth inhibition in comparison to untreated controls in 8/10 head & neck cancer PDX models. Complete tumor regression was observed in 30% (3/10) PDX models when treated with 1 mg/kg TR1801-ADC. 50% showed statistical significant partial regression while two models (20%) showed no significant tumor inhibition. Altogether, TR1801-ADC is a novel highly potent cMet ADC that shows very promising efficacy in PDX models of gastric, colorectal and head & neck cancer with long lasting anti-tumor effect with a single dose.

Citation Format: Marco Gymnopoulos, Oscar Betancourt, Vincent Blot, Ryo Fujita, Diana Ly, Sophie Nguyen, Jeanette Snedden, Jose Villicana, Jon Wojciak, Eley Wong, Neki Patel, Francois D'Hooge, Balakumar Vijayakrishnan, Conor Barry, John A. Hartley, Phil W. Howard, Roland Newman, Julia Coronella. TR1801-ADC, an optimized anti cMet PBD ADC with high efficacy in solid tumors of the GI tract and head & neck cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 225.

The Effect of Molecular Weight, PK, and Valency on Tumor Biodistribution and Efficacy of Antibody-Based Drugs

Poor drug delivery and penetration of antibody-mediated therapies pose significant obstacles to effective treatment of solid tumors. This study explored the role of pharmacokinetics, valency, and molecular weight in maximizing drug delivery. Biodistribution of a fibroblast growth factor receptor 4 (FGFR4) targeting CovX-body (an FGFR4-binding peptide covalently linked to a nontargeting IgG scaffold; 150 kDa) and enzymatically generated FGFR4 targeting F(ab)2 (100 kDa) and Fab (50 kDa) fragments was measured. Peak tumor levels were achieved in 1 to 2 hours for Fab and F(ab)2 versus 8 hours for IgG, and the percentage injected dose in tumors was 0.45%, 0.5%, and 2.5%, respectively, compared to 0.3%, 2%, and 6% of their nontargeting controls. To explore the contribution of multivalent binding, homodimeric peptides were conjugated to the different sized scaffolds, creating FGFR4 targeting IgG and F(ab)2 with four peptides and Fab with two peptides. Increased valency resulted in an increase in cell surface binding of the bivalent constructs. There was an inverse relationship between valency and intratumoral drug concentration, consistent with targeted consumption. Immunohistochemical analysis demonstrated increased size and increased cell binding decreased tumor penetration. The binding site barrier hypothesis suggests that limited tumor penetration, as a result of high-affinity binding, could result in decreased efficacy. In our studies, increased target binding translated into superior efficacy of the IgG instead, because of superior inhibition of FGFR4 proliferation pathways and dosing through the binding site barrier. Increasing valency is therefore an effective way to increase the efficacy of antibody-based drugs.

Selective activity against proliferating tumor endothelial cells by CVX-22, a thrombospondin-1 mimetic CovX-Body

CVX-22 is a CovX-Body, produced by covalently attaching a thrombospondin-1 (TSP-1) type 1 repeat peptide mimetic to a humanized IgG1 molecule. To dissect the antiangiogenic mechanism of CVX-22, the numbers and proliferative status of defined tumor endothelial cell (TEC) subsets from the B16 and C32 melanoma models were examined. CVX-22 treatment reduced the numbers of activated, vascular endothelial growth factor receptor 2 (VEGFR2)-positive TECs. Because the vast majority of mitotically active TECs reside in the VEGFR2 subset, a reduction in numbers of this compartment resulted in an 82% overall decrease in BrdU labeling of TEC. However, the rate of proliferation and VEGFR2 receptor density of this VEGFR2-positive subpopulation were unaffected. Instead, CVX-22 induced endothelial cell apoptosis both in vitro and in vivo, indicating that CVX-22 acts by selective deletion of activated, VEGFR2-positive TEC. The overrepresentation of activated cells in sites of tumor angiogenesis may confer a unique specificity of CVX-22 for tumor vasculature.