PK1 is a synthetic N-(2-hydroxypropyl)methacrylamide copolymer-doxorubicin (dox) conjugate currently undergoing Phase II evaluation in the United Kingdom. We have studied the activity of PK1 in three murine colon tumor models that differ in terms of morphology and vascularization in an attempt to determine which factors are most important in the tumor response to PK1. Vascular permeability was evaluated with Evans Blue, and pharmacokinetic studies in MAC15A and MAC26 used high-performance liquid chromatography to monitor both PK1 uptake and dox release in the tumors. Cathepsin B activity was assessed using a specific substrate. PK1 (40 mg x kg(-1) dox equivalent) was significantly more effective than dox alone (10 mg x kg(-1)) was against MAC15A tumors, which possess enhanced perfusion and retention, but not against MAC26 tumors, although MAC15A was also responsive to PK1 when grown as avascular micrometastatic deposits in the lung. Pharmacokinetic studies showed similar levels of PK1 in both tumors. Peak tumor levels of released dox were 7-fold greater in the responsive MAC15A tumor (53 microg x ml(-1)) compared with the less responsive MAC26 tumor (7.7 microg x ml(-1)) and more than 18-fold greater in MAC15A than when free dox was given. These differences in response correlated also with an increased lysosomal activity of cathepsin B. Calculated AUCs for intratumoral dox released were 431 microg x h x g(-1) and 775 microg x h x g(-1) for MAC15A and MAC26, respectively. These AUCs are 4-fold and 7-fold higher, respectively, than when dox is given alone. This study has shown that activity and the pharmacokinetics of PK1 and released dox are dependent on both the vascular properties and enzyme content of the tumors. These studies are likely to have clinical implications as aggressive tumors are known to have increased protease activity.