Cytokines induce thymidine phosphorylase expression in tumor cells and make them more susceptible to 5'-deoxy-5-fluorouridine

New targets for pyrimidine antimetabolites in the treatment of solid tumours. 1: Thymidylate synthase

Thymidylate synthase forms the target for anticancer therapy with fluoropyrimidines. Anticancer activity can be increased by the use of different modulators of fluoropyrimidine metabolism, which lead to an enhanced inhibition of thymidylate synthase. In vitro and in vivo studies with fluoropyrimidines and two of these modulators, folinic acid (leucovorin) and interferon, are summarized. The promise of these preclinical results is reflected by the response data of several clinical trials. The biochemical effects of these modulators are described and illustrated by the fluoropyrimidine-mediated inhibition of thymidylate synthase in tumour samples, which is clearly enhanced by folinic acid. The regulation of thymidylate synthase synthesis may also be crucial for total blockade of thymidylate synthase activity. This regulation may be influenced by interferon-gamma. Although the addition of modulators increases the activity of fluoropyrimidines at the level of thymidylate synthase, most solid tumours, especially colorectal carcinomas, are resistant to these combinations. For this reason, new, more potent inhibitors of thymidylate synthase have been developed, the antifolates. Preclinical data show that some of these compounds have good antitumour activity, but they still have to prove their value in the clinic. These two approaches, the use of modulators and new compounds, have shown activity preclinically and the extension of these findings to clinical studies stresses the importance of thymidylate synthase as a target in fluoropyrimidine therapy of solid tumours.

Biologic agents as biochemical modulators: pharmacologic basis for the interaction of cytotoxic chemotherapeutic drugs and interferon

Biochemical modulation of cytotoxic cancer chemotherapeutic agents is one means of enhancing the activity and selectivity of antitumor drugs. Traditionally this approach has utilized detailed information regarding a particular enzymatic reaction or biochemical pathway to develop potential modulating agents. In contrast, the reported clinical therapeutic activity of IFN in combination with cytotoxic agents has prompted a reexamination of the biochemical actions of the cytokine. Interferon elicits a number of cellular actions that might contribute to its pharmacologic activity, including both direct antitumor effects and host-mediated actions. The best understood are those related to the cytotoxicity of the fluoropyrimidine antimetabolites and include enzymatic reactions involved in fluoropyrimidine metabolic activation, catabolism, and interaction with its target enzyme. However, even in this instance, a mechanistic association of a specific pharmacologic action with therapeutic activity remains to be determined. These studies demonstrate that cytokines and other biologic agents may exert specific biochemical modulations that augment (or potentially attenuate) the activity of the cytotoxic chemotherapeutic agents.

Interferon alpha and 5'-deoxy-5-fluorouridine in colon cancer: effects as single agents and in combination on growth of xenograft tumours

Interferon-alpha (IFN-alpha) enhances the activity of the 5-fluorouracil (5-FU) prodrug 5'-deoxy-5-fluorouridine (5'-dFUrd) in colorectal cancer cells in vitro by upregulating the enzyme pyrimidine nucleoside phosphorylase (PNPase), which is responsible for converting 5'-dFUrd to 5-FU. We examined whether such enhancement also occurs in vivo using human colorectal xenografts in nude mice. The Co-115 line has high basal levels of PNPase and the enzyme level was increased in tumours from mice treated for 3 weeks with 50,000 IU/day (5 days/week) of IFN-alpha A/D. The prodrug 5'-dFUrd (200 mg/day, 5 days/week) had a much greater antitumour activity than 5-FU had when it was used at an approximately equitoxic dose (20 mg/day, 5 days/week). However, because of the high activity of 5'/dFUrd as a single agent, no enhancement by IFN-alpha A/D was observed. Studies on xenografts of WiDr cells indicated that this line is much less sensitive to 5'-dFUrd. However, treatment of animals with IFN-alpha A/D at doses of 75,000 IU/day or 150,000 IU/day resulted in significant inhibition of WiDr tumour growth. Combination treatment with 75 mg/kg/day or 150 mg/kg/day of 5'-dFUrd resulted in enhanced antitumour activity, particularly at the higher dose of IFN-alpha A/D. Synergy of this drug combination was confirmed by isobologram analysis. Analysis of PNPase levels in WiDr tumours, excised from mice treated with IFN-alpha A/D, demonstrated that the enzyme activity was increased by IFN-alpha in a dose-dependent manner. Slight increases were also seen in normal liver and intestine from the same animals. Our results indicate that modulation of converting enzymes for anticancer prodrugs by cytokines could be a novel therapeutic strategy for combination therapy of colorectal cancer.