Pharmcodynamic endpoints of toxicity following oral metronomic dosing of CPT-11 with ZD1839 or ZD6474 in mice

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Epidermal growth factor receptor (EGFR) and vascular endothelial growth factor receptor (VEGFR) are attractive targets for sensitizing tumors to chemotherapy, however, therapeutics against EGFR and VEGFR have marginal antitumor activity when used alone. Co-administration of ZD1839 (gefitinib), an EGFR inhibitor, with CPT-11 (irinotecan), a topoisomerase I inhibitor, has shown reasonable antitumor activity against colon and lung tumors but at doses near the maximum tolerated dose, resulting in severe side effects including diarrhea and neutropenia. CPT-11 is a substrate for the multidrug resistant transporter protein ABCG2, which can lead to resistance. As ZD1839 is also a substrate for ABCG2, it has been proposed that the side effects observed following ZD1839 plus CPT-11 are the result of ZD1839 altering efflux of CPT-11 in intestinal cells leading to increased levels in the gut. Lower doses given more frequently (metronomic dosing) may reduce side effects while retaining therapeutic effectiveness. ZD6474 (vandetanib) is a novel small molecule inhibitor of vascular endothelial growth factor receptor-2 (VEGFR-2) and EGFR, but does not alter CPT-11 pharmacokinetics in mice and thus appears to not interact with ABCG2. The aim of this study was to determine whether oral metronomic dosing of CPT-11 with ZD6474 or ZD1839 results in enhanced CPT-11 induced toxicity. Non-tumor bearing mice were administered CPT-11 at three dose levels (20, 40 or 60 mg/kg) alone or in combination with ZD1839 or ZD6474 (each at 25 mg/kg) p.o. qD x 4 days x 2 cycles. Significant weight loss (P < 0.02) was observed when CPT-11 (60 mg/kg) plus ZD1839 was compared to CPT-11 (60 mg/kg) alone or in combination with ZD6474. With the exception of ZD6474 plus CPT-11, all treatments resulted in significant intestinal toxicity, as determined via villus length to crypt depth ratio, compared to vehicle. Interestingly, mice treated with ZD1839 in addition to CPT-11 had enhanced toxicity (reduced villus length: crypt depth ratio) compared to CPT-11 alone (P < 0.001). CPT-11 and SN-38 levels were measured in the plasma, gut and liver 2.5 hr post last dose. While SN-38 levels were not significantly altered, plasma levels of CPT-11 following either ZD compound were significantly higher than without ZD6474 or ZD1839. Complete blood counts were unaffected by treatment. While immunohistochemistry analysis of intestinal proliferation (Ki-67) and apoptosis (active caspase-3) in the small intestine showed a decreasing and increasing trend, respectively, from vehicle to high dose CPT-11, neither ZD compound significantly altered CPT-11 induced changes. Overall, metronomic oral dosing of CPT-11 plus ZD6474 or ZD1839 was well tolerated in mice at the doses and schedules tested in this study. It also appears that ZD6474 does not enhance CPT-11 induced toxicity to the same extent as ZD1839.