Liposomes in the treatment of malignancy: a clinical perspective

Technological advances in liposomal preparation and efficient drug entrapment, along with supportive preclinical studies, have led to a number of recent clinical trials utilizing liposomes as drug carriers in the treatment of human malignancy. Although the results of these trials must be considered preliminary, it is clear that liposomal delivery of chemotherapeutic agents is safe at the doses administered. Aside from minor constitutional symptoms, virtually all toxicity could be attributed to release of the incorporated drug. Myelosuppression tends to be the dose-limiting toxicity with free drug, whereas constitutional symptoms are more likely to occur with encapsulated biologic therapy. Prior to human trials, there was fear that intravenous injection of liposomes could result in pulmonary emboli. No cases of pulmonary embolism secondary to liposome therapy have been recorded. The objective response rate in the patients studied appears to be minimal. This is not surprising, since the overwhelming majority of patients studied had disease that was advanced and previously shown to be refractory to therapy. Subgroups of patients that appear to benefit most include those with breast cancer who were treated with liposomal doxorubicin and those with advanced melanoma treated with liposomal tumor vaccines. Additional phase II and III clinical trials will better define the effectiveness of treatment modalities incorporating liposomes. VI-A. Future directions One of the earliest applications of liposomes may be in the amelioration of drug toxicity. Although not yet proven, the clinical studies reviewed suggest that liposomal delivery of doxorubicin reduces cardiotoxicity without sacrificing antitumor effect. Although similar claims have been made in support of continuous infusion doxorubicin [11], one can avoid unnecessary hospitalization or the bulk and expense of portable infusion devices by a single administration of the liposomal preparation. Liposome encapsulation can markedly alter the biodistribution and pharmacokinetics of well-known chemotherapeutic agents. The effectiveness of liposomal drug delivery in human trials thus far has probably been more closely related to altered pharmacokinetics rather than enhanced drug delivery to tumor or increased tumor responsiveness. As demonstrated by Gabizon [19], increased liposome circulating time in the murine model can be achieved by using small unilamellar vesicles containing a phosphatidylcholine of high phase-transition temperature and a small molar fraction of monosialoganglioside or hydrogenated phosphatidylinositol.(ABSTRACT TRUNCATED AT 400 WORDS)