A phase II trial to evaluate pemetrexed clinical responses in relation to tumor methylthioadenosine phosphorylase (MTAP) gene status in patients with previously treated metastatic urothelial carcinoma.

Abstract

385 Background: According to The Cancer Genome Atlas, 26.8% of bladder tumors harbor homozygous deletion of the MTAP gene, which encodes for an essential enzyme for the salvage pathway of nucleotide synthesis. Pemetrexed is a well-known anti-folate agent that inhibits the de novo pathway of nucleotide synthesis. Therefore, we hypothesized that tumors with MTAP gene loss have a synthetic lethality to pemetrexed due to loss of both essential nucleotide synthesis pathways. We proposed a study to determine pemetrexed response rate in patients with MTAP-deficient metastatic urothelial carcinoma (mUC). Methods: We carried out a single-arm, open-label, phase II clinical trial with pemetrexed in patients with mUC and MTAP loss by immunohistochemistry. This is an interim analysis of results. Each patient receives pemetrexed (500 mg/m2) on day 1 of each 21-day cycle. Pemetrexed is continued until progression of disease (PD) and a repeat biopsy is obtained at that time. Pre- and post-treatment blood, urine and tumor samples are obtained for dynamic monitoring of immune and molecular correlates to clinical activity. Results: Six patients have been enrolled on this trial and all have received at least one previous treatment including a cisplatin-containing regimen in 5 (83%) patients and an immune checkpoint inhibitor in 4 (66%) patients. Of these 6 patients, 1 (17%) had complete response (CR); two (33%) had partial response (PR); 1 (17%) had stable disease; 1 (17%) was not evaluable; 1 (17%) had PD. Treatment was generally well tolerated with mostly grade 1 or 2 adverse events. Only 2 of 6 patients developed grade 3 anemia. Immunologic and molecular analyses are ongoing on all collected samples. Conclusions: Our study show that in heavily pretreated MTAP deficient mUC patients, pemetrexed is still an effective and well-tolerated therapy with an objective response rate (ORR=CR+PR) of 50%, which is higher than previously reported pemetrexed activity (ORR=8-28%) as first or second line treatment for mUC. Clinical trial information: NCT02693717.