We recently demonstrated that the
cysteine proteinase inhibitor,
E-64, sensitizes human
Burkitt's lymphoma (Daudi) to the antitumor action of
bleomycin (BLM) by blocking its metabolism. We now report that
E-64 sensitizes the BLM resistant human lung
carcinoma A-549 by a mechanism unrelated to the inhibition of BLM metabolism. Treatment of A-549
tumor-bearing nude mice with either BLM (10 mg/kg) or
E-64 (40 mg/kg) every other day for 10 days did not inhibit
tumor growth. However, a 30 min pretreatment with
E-64 prior to BLM caused complete and sustained inhibition of
tumor growth. In contrast to our results with
Burkitt's lymphoma,
E-64 did not inhibit BLM metabolism but rather enhanced the
tumor accumulation of BLM; within 10 min of BLM administration,
tumors from
E-64 pretreated mice showed a 6-fold higher accumulation of BLM A2 compared to non-pretreated xenografts. Furthermore, the level of
tumor-associated BLM A2 remained 2-fold higher in
E-64 pretreated mice 20 and 30 min after BLM administration. In
E-64 pretreated mice, the plasma level of BLM was increased by 2-fold. These results demonstrate that the
cysteine proteinase inhibitor,
E-64, sensitized human lung
carcinoma A-549 to BLM and, contrary to the expected mechanism, this effect of
E-64 was not related to the inhibition of BLM metabolism.