Macrophage-mediated activation of camptothecin analogue T-2513-carboxymethyl dextran conjugate (T-0128): possible cellular mechanism for antitumor activity

Camptothecin (CPT) analogue T-2513-carboxymethyl (CM) dextran conjugate (T- 0128) suppressed human tumor xenografts that were refractory to CPTs. This improvement was explained by its altered pharmacokinetics, but the cellular mechanism of action is still not clear. For this reason, in the present st udy we examined the determinants of T-0128 action at the cellular level. In vitro tests showed that T-0128 was inactive, indicating that the requireme nt for its activity lies in the release of linked T-2513, accompanied by th e cellular uptake of the conjugate. The accumulation varied between cell li nes: tumor cells, including Walker-256 carcinoma and B16 melanoma, showed o nly a marginal uptake and an undetectable drug release in the medium. In co ntrast, macrophage-like cells, such as J774.1, internalized T-0128 very eff iciently, and liberated T-2513. With regard to the mode of accumulation, fl uid-phase pinocytosis seems to be a key factor based on the followings: a s imilar cell-specificity existed in the uptake of FITC dextran, a marker of fluid-phase pinocytosis. Also, the macrophage uptake of T-0128 increased al most linearly with its medium concentration and was insensitive to dextran sulfate, a ligand for macrophage scavenger receptor. Comparative efficacy s tudies of T-0128 in the presence and absence of macrophages demonstrated th at macrophages increased the efficacy of T-0128. The enhancement could be e xplained in terms of increases in the amount of released T-2513. Overall, t hese results lead us to the conclusion that T-0128 acts like a Trojan horse with the help of macrophages: T-0128 is taken up by macrophages in tumor t issues, and the liberated T-2513 kills tumor cells. (C) 2000 Elsevier Scien ce B.V. All rights reserved.