In Vitro and in Vivo Effect of Doxorubicin Combined with Liposome-encapsulated Muramyl Tripeptide on Canine Monocyte Activation

Abstract

Chemotherapeutic agents have been shown to enhance the antitumor activity of biological response modifiers and cytokines in rodents and humans. The purpose of this study was 2-fold: (a) to determine whether doxorubicin (DOX) would enhance or interfere with the effect of muramyl dipeptide and lipopolysaccharide on canine monocyte activation as measured by an in vitro WEHI-164 cell cytotoxicity assay; and (b) to evaluate the in vivo effect of DOX alone and combined with liposome-encapsulated muramyl tripeptide-phosphatidylethanolamine (L-MTP-PE) on monocyte activation and serum tumor necrosis factor activity. The in vitro results showed that increasing concentrations of DOX for either 1 or 24 h incubation did not directly enhance or inhibit spontaneous or activated monocyte supernatant-mediated cytotoxicity. The in vivo study showed that monocyte supernatant-mediated cytotoxicity was increased on day 3 and significantly elevated on day 7 (P = 0.016) post-DOX (30 mg/m², single injection) administration. When DOX was given in combination with L-MTP-PE (2 mg/m², twice weekly for 3 weeks), monocyte-mediated cytotoxicity was enhanced on days 3 through 10 with a significant increase on day 10 (P < 0.001). In vivo monocyte supernatant-mediated cytotoxicity was significantly elevated in dogs receiving L-MTP-PE alone at 2 h after day 0, 7, and 14 treatment, and this response was further enhanced by DOX. Serum tumor necrosis factor activity at 2 h post-L-MTP-PE was enhanced and sustained for a longer period of time in dogs that also received DOX. We conclude that DOX administered with L-MTP-PE will enhance canine monocyte activation induced by DOX or L-MTP-PE alone, and suggest that DOX may be combined with L-MTP-PE early in the treatment of cancer patients.