Multicycle Pretargeted Radioimmunotherapy using 177Lu for Intraperitoneal GPA33-Expressing Colorectal Xenografts

290 Objectives: Peritoneal carcinomatosis (PC) occurs when tumor spreads to the peritoneal lining, and is seen in the end stage of GI malignancies, such as colon cancer. PC is considered incurable and development of curative therapies is an ongoing need. Previously, we have reported Pretargeted Radioimmunothearpy (PRIT) methods in mouse xenografts from GPA33-expressing human CRC SW1222, using IV injection of 177Lu radiohaptens targeting GPA33, a cell-surface glycoprotein on >95% of CRC1,2, via a bispecific antibody (BsAb). In prior work, we achieved cure in a multicycle flank xenograft model3 and prolonged survival without cure in a single cycle IP model of PC.1 We attempt to improve outcomes in our IP PC model using a multicycle PRIT regimen with assessment of acute and chronic toxicity. Methods: 6 wk old female athymic nude mice were inoculated IP with 5E6 cells of a stably luciferase/GFP-transfected SW1222 in 200µL media. Mice were screened with BLI on day 24 and included if BLI >5E5p/s/cm2/sr. Mice were grouped to no treatment (n=6), HuA33-C825 BsAb (A33-BsAb) (n=6), [177Lu]Lu-DOTA-Bn (Non-Targeted) (n=8), and HuA33-C825 BsAb + [177Lu]Lu-DOTA-Bn (A33-Targeted) (n=8). 3 cycles separated by 1 wk were administered starting on days 27, 34, and 41. On cycle day 1, the mice were injected IP with 0.25 mg (1.19 nmol) BsAb. On cycle day 2, 25 µg (2.76 nmol) CCA16-DOTAY clearing agent (CA) was given IV 22 hrs from BsAb. Mice in therapy groups were injected IP with 3 mCi (600 pmol) [177Lu]Lu-DOTA-Bn 4 hrs after CA. Weekly BLI was performed, and ROIs of the IP cavity created. Weekly hematology and weight measurements were taken for toxicity monitoring. Animals were euthanized if weight loss was >20% baseline or if moribund with severe abdominal distension. Results: At 183 days, 6/8 A33-Targeted mice were alive and with no apparent clinical disease compared to 1/20 cumulative control mice. This mouse was noted to have a large progressing flank tumor that was at the inoculation site with minimal IP disease. A33-Targeted deaths occurred due to toxicity/weight loss (>20% baseline) (n=1) and progression of disease following initial response (n=1). 1 death in the Non-Treatment group was secondary to toxicity/weight loss, and all other deaths in the remaining groups due to progression of disease. Median Survival (days) were 44 for No Treatment, 52.5 for A33-BsAb, 42 for Non-Targeted, and not reached after 183 days for A33-Targeted not reached (Log-Rank Mantel Cox test, p=0.0004). Median Survival of prior 1 cycle A33-Targeted was 84.5 days (Log-Rank Mantel Cox Test, p=0.016). Neutropenia occurred in all therapy groups (nadir: post treatment day (ptd) 23). WBC recovered to low normal by ptd 30, however remained lower than no therapy controls. All other hematological parameters remained within reference ranges. BLI was comparable at baseline in all groups (ANOVA, p=0.984) with increasing signal in all groups till day 40 (2 days prior to cycle 3) at which point A33-Targeted signal began to decline. On day 68, A33-Targeted BLI was below baseline while all other groups continued to be elevated. A33-Targeted BLIs continued to decrease in all but one mouse, which began increasing on day 68. Final BLI in A33-Targeted Therapy was reduced by 4.38E6from matched baseline in surviving mice (2-tailed matched t-test, p=0.015). Conclusions: Multicycle PRIT using anti-GPA33 BsAb targeting IP SW1222 resulted in significantly prolonged survival compared to 1 cycle administration or 3 cycle controls with minimal toxicity and weight loss. BLI was successful in monitoring response. For A33-Targeted mice, median Survival was not reached, and they appear clinically disease free with possible complete response vs indolent disease in 75% of mice. Final histopathology of remnant calcified tumor nodules and chronic toxicity is ongoing. The anti-GPA33 DOTA-PRIT system utilizing 177Lu is a promising theranostic treatment approach for otherwise incurable colorectal peritoneal carcinomatosis.