Tumor drug distribution and target engagement of MLN9708, an investigational proteasome inhibitor, in patients with advanced solid tumors.

Abstract

3077 Background: MLN9708 is a potent investigational proteasome inhibitor, which upon intravenous (IV) administration immediately hydrolyzes to the active form MLN2238. MLN9708 is currently being evaluated in a phase 1 trial in solid tumors (NCT00830869). This trial has a dose-escalation arm and five expansion cohorts: non-small cell lung cancer (NSCLC), soft tissue sarcoma, head and neck cancer, prostate cancer, and a tumor biopsy cohort of mixed histology. The purpose of the tumor biopsy cohort was to obtain pre- and post-dose biopsies to determine drug distribution and target engagement in post-dose tumor samples. The latter was measured by the increase in levels of ATF-3, a marker of unfolded protein response/endoplasmic reticulum stress, which is upregulated in response to proteasome inhibition. Methods: The tumor biopsy cohort included 20 patients dosed at the maximum tolerated dose who consented to core needle biopsies during screening and after either the first or second dose of MLN9708 (IV 1.76 mg/m2; 4–20 hours post-dose). Tumor biopsies were individually weighed, homogenized, and analyzed for the presence of MLN2238 using a quantified LC/MS/MS methodology. ATF-3 levels in tumors were determined by an immunohistochemical assay (IHC) on six sections for each tumor biopsy. Tumor area was identified using Aperio Genie, a machine learning program for pattern recognition, and the percentage of ATF-3 positive area in the tumor was measured. Results: Biopsies from 20 patients were collected for assessment of drug distribution and target engagement. Ten patients with paired pre- and post-dose biopsies of sufficient size were considered evaluable for PK analysis; MLN2238 was present in all 10 (100%) post-dose biopsies analyzed. Tumor pairs from 7 patients passed quality control by H&E staining for tumor content and were evaluable for ATF-3 IHC. Six of 7 paired samples (86%) showed a statistically significant (p<0.05) increase in post-dose ATF-3 levels. Conclusions: Overall, emerging data from MLN9708 phase 1 solid tumor analysis show that MLN2238 is present in tumors and demonstrates target engagement upon inhibition of the proteasome in tumor tissue biopsies.