Background: Peritoneal implantation of mouse renal adenocarcinoma cell-containing (RENCA) Macrobead (MB) represents a cell-system-based approach to the treatment of advanced, mCRC that has been evaluated to date in Phase IIa trials. The data indicate that there are “responders” (R) and “non-responders”(NR) as reflected in overall survival (OS), where “response” is defined as a >20% decrease in either/both CEA or CA19-9 during the first 30 days after MB implantation. We analyzed whether the “response” is due to a post-implant systemic inflammatory response (SIR) or rather a direct inhibitory effect of the MB. Methods: Thirty-four treatment-resistant mCRC patients (pts) were implanted laparoscopically at least once with RENCA MB. Pts were considered R (n=25), or NR (n=9), based on tumor marker responses within the first 30 days. CRP, IL-6, TNF-alpha, and ESR, as measures of SIR, were measured at Day 14 and 30. Results: All 34 pts showed SIR to MB implantation, as indicated by transient rises in CRP, IL-6, TNF-a, and ESR. Baseline CRP values (R, mean 3.24+/-4.39 vs. NR, 2.96+/-3.43; t-test, p=0.86), Day 14 CRP values (R, mean 20.97 +/- 7.21 vs. NR, 14.5+/-8.78; t-test, p=0.04), Day 30 CRP values (R, mean 8.21+/-5.43 vs. NR, 10.76+/-6.92; t-test, p=0.27) and mean changes in IL-6 (baseline p=0.28; Day 14 p=0.36; Day 30 p=0.54), TNF-a (baseline p=0.37; Day 14 p=0.32; Day 30 p=0.29) did not show statistically significant differences between R and NR groups. Conclusions: Data suggest that early tumor marker decreases in R of RENCA MB are likely not due to the induced SIR, but rather a possibly direct anti-tumor-cell effect by factors released by MB. This supports the importance of the MB-induced changes in the MEF-2 pathway in the target colorectal cancer cell/tumor reported previously. Studies of clinical efficacy of MB continue in a Phase IIb clinical trial. Clinical trial information: NCT01053013