Emerging roles of targeted small molecule protein-tyrosine kinase inhibitors in cancer therapy

Targeted protein-tyrosine kinase inhibitors (PTKIs) comprise a new, rapidly evolving class of low molecular weight anticancer drugs. Two members of this class, imatinib (Gleevec(R)) and gefitinib (Iressa(R)), are currently approved for market use in the United States. This review discusses the scientific history behind these two PTKI drugs, including the role of the targeted kinase in cancer etiology, the biochemistry of selective inhibition, the evaluation of clinical efficacy, and the mechanisms whereby drug resistance has emerged. Other PTKIs undergoing clinical evaluation are also described, including epidermal growth factor receptor kinase inhibitors (erlotinib, PKI166, and CI-1033) and PTKIs designed to disrupt tumor vascularization (SU5416, SU6668, SU11248, PTK787, and ZD6474). How might one apply current knowledge to the efficient development of new agents that would target as-yet-unexploited oncogenic PTKs such as chimeric anaplastic leukemia kinases or Janus kinases? Ideally, the targets should contain structurally distinct drug interaction epitopes, although it is not necessary that these epitopes be unique to a single target, because effective drugs may inhibit multiple kinases involved in an oncogenic process. Oral availability is a highly desirable feature because daily oral administration can maintain a sustained efficacious plasma concentration, whereas intermittent parenteral administration may not. Perhaps most importantly, one must verify the presence of an appropriate molecular target on, a case-by-case basis before selecting a patient for PTKI therapy. Thus, the development of molecularly targeted diagnostic tools will be crucial to the ultimate success of molecularly targeted PTKI therapy.