Antitumor activity of erlotinib in combination with capecitabine in human tumor xenograft models

Purpose: To examine the antitumor activity and tolerability of a combination comprising erlotinib and capecitabine in human colorectal, breast and epidermal cancer xenograft models. Further aims of the study were to examine the effects of single-agent erlotinib therapy on tumor growth, and on thymidine phosphorylase (TP) and dihydropyrimidine dehydrogenase (DPD) levels, (enzymes which activate and deactivate capecitabine, respectively) in tumor tissue. Methods: BALB/c nu/nu mice bearing LoVo and HT-29 (colon cancer), A-431 (vulval cancer), and KPL-4 and MAXF401 (breast cancer) human tumors were treated with erlotinib 100 mg/kg/day and/or capecitabine 359 or 90 mg/kg/day, by oral administration once daily for 14 days. Results: The maximum tolerated dose (MTD) of erlotinib, formulated in carboxymethylcelluose/Tween 80, was identified as 125 mg/kg/day. Erlotinib at a dose of 100 mg/kg/day achieved significant tumor-growth inhibition in the, LoVo, KPL-4, and A-431 models. Some inhibition of MAXF401 tumor growth was observed, but was not significant. In the HT-29 model, erlotinib showed less marked but statistically significant antitumor activity. On day 15, mean tumor-growth inhibition in HT-29, LoVo, KPL-4, MAXF401, and A-431 models was 46, 74, 71, 20, and 85%, respectively. Evaluation of erlotinib/capecitabine combination therapy, at sub-optimal doses, in the three erlotinib-sensitive tumor models LoVo, KPL-4 and A-431, demonstrated at least additive activity with the combination compared with the single agents. In the A-431 and LoVo models, the combination of agents had greater antitumor activity than the single agent capecitabine alone at the MTD. Erlotinib in combination with capecitabine was not associated with significantly increased toxicity compared with single-agent therapy. Erlotinib 100 mg/kg/day induced significant upregulation of TP and DPD in the LoVo model, a significant upregulation of TP in the HT-29, MAXF401 and A-431 models, but had no obvious effect on TP and DPD levels in the KPL-4 model. In the A-431 model, selective upregulation of TP by erlotinib 100 mg/kg resulted in an increased TP:DPD ratio. In the LoVo model, immunohistochemistry revealed marked upregulation of TP (but not DPD by erlotinib). Conclusions: Erlotinib inhibits tumor growth in a range of human tumor xenograft models, including breast and colorectal cancer (CRC). Erlotinib and capecitabine demonstrated at least additive activity in LoVo, KPL-4 and A-431 tumor models. The antitumor activity of the combination was greater than that of capecitabine alone at the MTD. Erlotinib treatment did affect the TP in the CRC tumor models as confirmed immunohistochemically. The findings of this study support clinical evaluation of erlotinib, both as a single agent and in combination with capecitabine, for the treatment of CRC and breast cancer.