Abstract 2431: In vivo antibody binding to tumor in xenograft rodent models and colorectal cancer patients treated with anti-A33 antibody KRN330
Authors: Berlin Jordan D. , Infante Jeffrey R. , Taguchi Eri , Goff Laura W. , Jones Suzanne F. , Chan Emily , Bendell Johanna C. , Rothenberg Mace L. , Burris Howard A. 
Published: February 2011

Abstract
Purpose:
Many antibody therapies have been introduced into clinical practice in the past decade. Although pharmacokinetic data provide information regarding serum concentration, there are few reports that show the interaction of antibodies with the tumor in vivo. KRN330 is a novel recombinant human IgG1 monoclonal antibody (mAb) targeting the A33 surface differentiation antigen that is uniformly expressed on the surface of 95% of colorectal cancer (CRC) cells. We investigated whether KRN330 binds and stays bound to tumors established in a xenograft rodent model as well as biopsied tumors from CRC patients treated with KRN330 on a phase I trial.
Methods:
Time-course of KRN330 binding was analyzed using a rodent xenograft tumor model bearing human LS174 colon cancer cells. Tumors were collected at specified time points, ranging from the end of infusion on Day 1 to Day 36 after a single dose of KRN330 (1 mg/kg). KRN330 on the tumor was detected with either flow cytometry or an immunohistochemical method (IHC) for each time point.
Clinical KRN330 binding data were analyzed using A33-positive tumor biopsies from 4 patients with advanced CRC who consented to be enrolled into the Phase 1 monotherapy study of KRN330. The biopsy was conducted on Day 8-9, or Day 14, after an initial 3 mg/kg dose of KRN330. KRN330 binding on human CRC tissues was assessed using IHC with the anti KRN330 idiotype antibody.
Serum concentrations of KRN330 were obtained at several time points in both studies.
Results:
Xenograft KRN330 binding analyses indicate that KRN330 rapidly bound to the tumor after dosing and remained bound throughout the study period. Although the serum concentration decreased to 15% of that originally observed at 10 minutes post administration by Day 8, based on flow cytometry data, the KRN330 bound on tumors on Days 8 and 15 was 100% and 70%, respectively, of the maximum level bound (Day 2). Xenograft tumors in animals that were infused with a non-specific human immunoglobulin G control showed there was very minimal binding on Day 2. IHC results demonstrated that staining intensity supported the flow cytometry analysis and remained at a similar level for up to Day 15.
Clinical binding data confirmed KRN330 bound and remained bound to the CRC tumor cells even at Day 14 as well as Days 8 and 9. On the other hand, serum concentrations of KRN330 decreased to 12% and 4% of the value at the end of infusion on Day 8-9 and Day 15, respectively.
Conclusion:
These data are the first to show that KRN330 binds rapidly and stays bound to CRC tumors for an extended period in rodent xenograft models as well as CRC patients. This suggests that KRN330 might be able to remain active on tumor even after the serum concentration drops to very low levels. Further investigation of this mAb in clinical studies is currently underway.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2431.