Aspects of cytotoxic drug penetration, with particular reference to anthracyclines.

Experimental data, particularly derived from tumour spheroids, indicate that drug penetration barriers may be an important determinant of cytotoxic drug efficacy, even in spheroids of only a few hundred microns in diameter. Clinically, tumour masses of this size would equate with those micrometastases which are the target of adjuvant chemotherapy in a wide range of tumour types. It is apparent, therefore, that even at this relatively early stage of the metastatic process, which ultimately proves to be fatal in many patients, measures aimed at improving drug penetration may prove to be crucial in improving the therapeutic efficacy of cytotoxic agents.