Abstract Background: The base excisional repair (BER) pathway plays an important role in the development of resistance to alkylating agents. TRC102 (methoxyamine HCL) inhibits BER by binding apurinic/apyrimidinic sites, which increases temozolomide (TMZ)-induced DNA strand breaks and apoptosis. We conducted a multihistology phase I/II trial of TRC102 in combination with TMZ in patients (pts) with refractory solid tumors (NCT01851369); here we report the clinical activity of this combination in the phase II refractory colorectal carcinoma (CRC) cohort. The phase I portion of the study demonstrated anemia to be the major dose limiting toxicity of this combination; the recommended phase II doses (RP2D) for the combination were: 125 mg TRC 102 + 150 mg/m2 TMZ (PO, D1-5 of 28-day cycles). The expanded phase II trial utilized a 3-arm Simon 2-stage design based on objective responses documented during the phase I portion of the study (CRC, NSCLC, and granulosa cell ovarian cancer). Methods: Phase II pts were treated at the RP2D, except that pts with BSA < 1.6 m2 received 100 mg of TRC102. Eligibility for the CRC cohort included pts with recurrent, metastatic, histologically confirmed colorectal adenocarcinoma after ≥ 2 lines of therapy, adequate organ function, and an ECOG performance status (PS) ≤ 1. Overall response rate (ORR) was defined as a partial response (PR) or a complete response (CR), per RECIST 1.1 criteria. Results: During the phase I trial, one pt with KRAS mutant CRC experienced a sustained PR of 21 cycles (maximum 70% decrease by RECIST). This pt decided to stop treatment due to travel constraints, but at 17 months off treatment had not progressed. Based on this response, 16 pts were enrolled in the CRC cohort of the phase II trial (3 rectal, 13 colon), all heavily pretreated (median: 6 lines of prior therapy, range: 3-14). Median pt age was 64 years (range: 46-80 years); all had an ECOG PS of 1. Median time on study was 2 cycles (range: 1 to 5 cycles) including 5 pts who came off study for early clinical progression. ORR for this cohort was 6% (1 PR). Nine pts had KRAS mutant disease, including the partial responder, who also was the only pt (of the 10 tested by tumor IHC) with MGMT-negative disease. Grade 3 and 4 protocol treatment-related toxicities included thrombocytopenia (2 pts), hemolysis (1 pt), and diarrhea (1 pt). No pts discontinued the protocol because of toxicity and only 1 pt required dose reduction (thrombocytopenia). Conclusions: Despite a promising phase I result from a pt with CRC, a phase II study of combined TRC102 and TMZ treatment in patients with metastatic colon or rectal adenocarcinoma displayed an ORR of 6%. Assessing tumor MGMT status may help identify pts who could benefit from this combination and pharmacodynamic studies are underway to assess drug activity in responding and non-responding patients. Supported in part by NCI Contract HHSN261200800001E. Citation Format: Geraldine OSullivan Coyne, Cecilia Monge Bonilla, Jennifer Zlott, Naoko Takebe, Robert Meehan, Lamin Juwara, Richard Piekarz, Laurence Rubinstein, Deborah F. Wilsker, Robert J. Kinders, Ralph E. Parchment, Shahanawaz Jiwani, Shivaani Kumar, James H. Doroshow, Alice P. Chen. A Phase II trial of TRC102 (methoxyamine HCl) in combination with temozolomide in patients with relapsed metastatic colorectal carcinoma [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 LB-293.