Molecular mechanisms of polyamine analogs in cancer cells.

The natural polyamines are aliphatic cations with multiple functions and are essential for cell growth. Soon after the critical requirement of polyamines for cell proliferation was recognized, the metabolism of polyamines was pursued as a target for antineoplastic therapy. Initially, much attention was focused on the development of inhibitors of polyamine biosynthesis as a means to inhibit tumor growth. The best-characterized inhibitor is alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase. While compensatory mechanisms in polyamine metabolism reduce the effectiveness of DFMO as a single chemotherapeutic agent, it is currently undergoing extensive testing and clinical trials for chemoprevention and other diseases. There has been increasing interest over the last two decades in the cytotoxic response to agents that target the regulation of polyamine metabolism rather than directly inhibiting the metabolic enzymes in tumor cells. This interest resulted in the development of a number of polyamine analogs that exhibit effective cytotoxicity against tumor growth in preclinical models. The analogs enter cells through a selective polyamine transport system and can be either polyamine antimetabolites that deplete the intracellular polyamines or polyamine mimetics that displace the natural polyamines from binding sites, but do not substitute in terms of growth-promoting function. Synthesis of the first generation of symmetrically substituted bis(alkyl)polyamine analogs in the mid-1980s was based on the theory that polyamines may utilize feedback mechanisms to auto-regulate their synthesis. In the 1990s, unsymmetrically substituted bis(alkyl) polyamine analogs were developed. These compounds display structure-dependent and cell type-specific cellular effects and regulation on polyamine metabolism. More recently, a novel class of analogs has been synthesized, which include conformationally restricted, cyclic and long-chain oligoamine analogs. The development and use of these analogs have provided valuable information for understanding the molecular mechanisms of targeting the polyamine pathway as a means of cancer therapy.