Relationship between reductive drug metabolism in tumour tissue of anthracyclines in microspherical form and anti-tumour activity.

Increased activity against a rat solid tumour of doxorubicin incorporated into protein microspheres and administered intratumourally was associated with both increased duration of exposure of tumour tissue to native drug and anaerobic bioreduction of doxorubicin to 7-deoxyaglycones, indicating formation of reactive drug intermediates within tumour tissue. To investigate which of these aspects of drug disposition determined activity we have compared the in vivo fate (clearance from and metabolism by tumour tissue) of doxorubicin in microspherical form with the analogue 4'-deoxydoxorubicin and related this to the tumour growth delay recorded for these drugs. Within the dose range 42 to 55 micrograms, growth delay (14-18 days) of doxorubicin in microspherical form was markedly superior to drug in solution, whereas growth delay of 4'-deoxydoxorubicin in microspherical form (4.3-7.2 days) was not greater than drug in solution. Metabolism to 7-deoxyaglycones by tumour tissue was not a prominent feature of either drug when administered in solution. However, in microspherical form both drugs were extensively metabolized (peak concentrations: 3.6 micrograms/g doxorubicin 7-deoxyaglycone; 2.5 micrograms/g 4'-deoxydoxorubicin 7-deoxyaglycone). Native drug concentrations in tumour tissue were similar after administration in microspherical form at 48 hr (doxorubicin 3.8 micrograms/g; 4'-deoxydoxorubicin 3.7 micrograms/g) and 72 hr (doxorubicin 2.4 micrograms/g; 4'-deoxydoxorubicin 2.7 micrograms/g). At both time points, following administration in microspherical form, tumour tissue concentrations of doxorubicin were significantly greater than when drug was administered in solution, whereas no significant differences were observed for 4'-deoxydoxorubicin. The results are inconsistent with the process of anaerobic bioreduction of doxorubicin to 7-deoxyaglycones being an important component of its anti-tumour activity in microspherical form and point to the importance of increased duration of exposure to native drug.