Antifolates targeted specifically to the folate receptor

Most antifolate drugs are efficiently transported by the reduced-folate carrier (RFC). However, several also bind with high affinity to the alpha-isoform of the folate receptor (alpha-FR) and there is evidence to suggest that this transport mechanism may contribute to their activity when the receptor is highly overexpressed or when the extracellular folate concentration is very low. In particular, the presence of the alpha-FR on tumour cell lines sensitises them to brief exposures to ZD9331. Nevertheless, it is the ubiquitous expression of the RFC in normal tissues that reduces patient tolerability to antifolate drugs. The overexpression of the alpha-FR in some epithelial tumours and its restricted distribution in normal tissues suggests an opportunity for the development of antifolates specifically targeted at alpha-FR overexpressing tumours. Potent cyclopenta[g]quinazoline-based inhibitors of thymidylate synthase (TS) have been discovered with high and low affinity for the alpha-FR and RFC, respectively. This class of agent is represented by CB300638 (TS Ki=0.24 nM) that displays high potency (IC50 approximately 3 nM) for A431-FBP cells (transfected with the alpha-FR) and KB cells (constitutive overexpression). Importantly, this activity is approximately 300-fold higher than for alpha-FR negative cell lines such as A431. In mice bearing the KB tumour xenograft we have demonstrated localisation of CB300638 to tumour and, more importantly, specific inhibition of TS in tumour and not in normal tissues. Data supports the clinical development of this class of agent with the prediction that toxicity would be reduced compared with conventional antifolate drugs. There are a number of challenges to this development posed by the uniqueness of the compounds and these are discussed.