RT Journal Article SR Electronic T1 Therapeutic efficacy of 225Ac-labeled nimotuzumab in a mouse model of EGFR positive KRAS-mutant metastatic colorectal cancer (mCRC) JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 233 OP 233 VO 61 IS supplement 1 A1 Khan, Behlol A1 Khan, Musharraf A1 Causey, Patrick A1 Perron, Randy A1 Gendron, Denise A1 Karkare, Sharayu A1 Parada, Angel C. A1 Geyer, Clarence R. A1 Fonge, Humphrey YR 2020 UL http://jnm.snmjournals.org/content/61/supplement_1/233.abstract AB 233Background: Metastatic colorectal cancer (mCRC) is the second leading cause of death from cancer with a 5-year survival rate of < 10% (stage IV), likely because over 45% of CRC patients have metastatic disease at initial diagnosis. About 80% of CRC patients overexpress EGFR. Mutations in the KRAS oncogene (present in 40% of mCRC) lead to constitutive over-activation of EGFR and drive de novo resistance to anti-EGFR drugs. In EGFR-positive mCRC patients with wild-type KRAS, the addition of anti-EGFR antibodies (e.g. cetuximab and panitumumab) to chemotherapy results in small albeit significant improvements in survival; there is no observed benefit in patients with KRAS mutations. Here, we propose to target KRAS-mutant EGFR positive mCRC using 225Ac-labeled nimotuzumab, a humanized anti-EGFR monoclonal antibody with affinity-optimized binding characteristics. Methods: We synthesized eight-membered macrocyclic bifunctional chelator p-Bz-SCN-macropa for conjugation with nimotuzumab and rituximab (control), followed by radiolabeling with 225Ac for alpha particle therapy. The radiochemical yield of 225Ac-nimotuzumab and 225Ac-ritxumab was >95%. The radioimmunoconjugates were developed and characterized by flow cytometry, immunoreactivity, radioligand binding assays, HPLC and internalization rate (live-cell imaging). We have evaluated the in vitro cytotoxicity in EGFR positive KRAS mutant colorectal cancer cell lines DLD-1, HT-29, CCL-227 and CCL-250 with different levels of EGFR density. We have also evaluated the in vivo efficacy of 225Ac-nimotuzumab in mice bearing DLD-1-iRFP-702 tumors and monitored tumor growth using near infrared (NIR) imaging. Mice were treated with three doses of 350 nCi/dose administered 10 days apart. In vivo study endpoint was tumor volume >= 1500 mm3. Results: Flow cytometry showed nearly > 95% binding to the cells. In all three EGFR positive colorectal cell lines (DLD-1, CCL-227, CCL-250) in vitro studies showed enhanced cytotoxicity of 225Ac-nimotuzumab compared with controls. HT29 had very low EGFR density/cell. IC50 in DLD-1 cell line: 225Ac-nimotuzumab (2.8 nM) < 225Ac-ritxumab (4.7 nM) < nimotuzumab (65 nM) < rituximab (305 nM). Similar trend was observed in the other KRAS-mutant mCRC cell lines. 225Ac-nimotuzumab prolonged the survival of mice bearing DLD-1 tumors compared with control antibody or non-treated controls. Median survival of DLD-1-iRFP-702 tumor bearing mice was 40 days and 34 days for non-treated and control 225Ac-rituximab treated mice, respectively, while median survival in 225Ac-nimotuzumab group had not been reached > 90 days since the start of treatment. There were no treatment related toxicities. Conclusions: 225Ac-nimotuzumab shows very promising outcomes in KRAS-mutant mCRC models and warrants further investigation. Additional studies in other xenograft models are ongoing. Funding: This work is funded by the Canadian Institute of Health Research (CIHR) grant # 408132