The safety and efficacy of PEN-221 somatostatin analog (SSA)-DM1 conjugate in patients (PTS) with advanced GI mid-gut neuroendocrine tumor (NET): Phase 2 results

Abstract

Background: PEN-221 is a small molecule drug conjugate composed of a SSTR2 binding somatostatin analog linked to the toxin DM1. PEN-221-001 was a study which assessed the safety, tolerability, pharmacokinetics (PK), and preliminary efficacy of PEN-221 in well differentiated neuroendocrine tumors (NETs) and small cell lung cancer. Here we present the efficacy outcomes for patients enrolled in the GI mid-gut cohort and the safety data for the entire study. Methods: Pts with advanced, SSTR2+ (by imaging) GI mid-gut NETs were enrolled in this cohort of the study. The primary objective was to determine the safety and efficacy of PEN-221 given intravenously, every (q) 3 weeks in patients with documented radiographic progression within the 6 months prior to enrollment. Patients previously treatment with cytotoxic chemotherapy were excluded. Preliminary efficacy was assessed using RECIST 1.1. A clinically meaningful efficacy result was defined as a Clinical Benefit Rate (CBR) > 75% and a median progression-free survival (mPFS) > 8 months. Results: 32 patients (17M/15F) were enrolled between January 2018 to June 2020 and the data cut-off for this report is July 31, 2020. The first nine patients were treated at the phase 1 determined Maximum Tolerated Dose of 18 mg. After review of the safety, tolerability, and PK data from these pts, the regimen was amended to 8.8 mg/m2 for all subsequent pts to achieve more uniform exposures (AUC) across all pts and reduce toxicity in pts with lower body-surface areas (BSA). The mean number of cycles received by pts in this cohort was 7 (range 1-18), with 5 pts still on treatment at time of data lock. PEN-221 was well tolerated in all pts at the dose of 8.8 mg/m2. The most frequent (≥20% pts) PEN-221 related adverse events of any grade were fatigue (39%), nausea (38%), diarrhea (35%), decreased appetite (30%), infusion reaction (24%), AST/ALT/Alk Phos increase (24%), and peripheral neuropathy (21%). Only 14 (10%) of these events were grade 3 or greater. Grade 3 PEN-221 related adverse events which were reported in 2 or more pts included fatigue (7.6%), ALT/AST/Alk Phos increase (7.6%), and peripheral neuropathy (3%). PEN-221 plasma median t1/2 was ̃4.5 h, with exposures uniform using BSA based dosing. Of the 26 pts who were evaluable for response, 23 (88.5%) had stable disease (SD) reported as their best response with a CBR of 88.5%. Target lesion shrinkage was observed in 10 (38%) patients. The median PFS for this cohort was 9 months (CI 5 – 16.5 months). Tumor marker data (Neuron Specific Enolase, Chromogranin A, 5-Hydroxyindoleacetic Acid, and Circulating Tumor Cells) will also be presented. Conclusions: PEN-221 appears well tolerated at 8.8 mg/m2 q 3 weeks and has demonstrated efficacy exceeding its clinical efficacy goals with a CBR of 88.5% and a mPFS of 9 months. A randomized trial of PEN-221 in GI mid-gut NET patients is now in development.