Raltitrexed increases tumorigenesis as a single agent yet exhibits anti-tumor synergy with 5-fluorouracil in ApcMin/+ Mice1

Abstract

The thymidylate synthase (TS) inhibitors raltitrexed (RTX) and 5-fluorouracil (FUra) have shown promising anti-tumor activity in preclinical and clinical settings for the treatment of colorectal cancer (CRC). Though the effects of these two agents have been reasonably well-characterized in cell lines, knowledge of their modes of action in vivo is limited. Here, we utilize the Apc Min/+ mouse, an animal model of intestinal tumorigenesis, to study the effects of RTX treatment alone and in combination with FUra. Rather surprisingly, RTX monotherapy resulted in a dose-dependent 4-10-fold increase in tumor number. The majority of these adenomas (74-95%) were rather small (i.e., less than 1 mm in diameter) and exhibited loss of heterozygosity at the Apc locus, suggesting an increase in mutational events leading to tumor development. RTX augmented BrdU-labeling of crypt epithelial cells, and retarded the movement of these cells along the crypt-villus axis. Co-administration of FUra and RTX resulted in a significant reduction in tumor number compared to mice treated with either RTX or FUra alone (P < 0.0001). In addition, FUra abrogated the RTX-mediated increase in BrdU labeling. In all, the results show that RTX increases tumor burden in the Apc Min/+ mouse, yet enhances the anti-tumor effect of FUra. This is the first illustration of in vivo synergy of RTX and FUra in a genetically predisposed animal model. Possible mechanisms underlying the current observations are discussed.

The Wnt/β‐catenin/T‐cell factor 4 pathway up‐regulates high‐mobility group A1 expression in colon cancer

“…It is well established that mutations in the APC tumour suppressor gene are responsible for the earliest stages of colorectal tumorigenesis . Similarly, we show that high levels of HMGA1 mRNA and protein are present in gastrointestinal tissues of APC Min/+ mice and that recovery of APC function in HT‐29 cells represses HMGA1 mRNA and protein levels.…”

Induction of intrachromosomal homologous recombination in human cells by raltitrexed, an inhibitor of thymidylate synthase

“…These putative recombination events were not studied at the nucleotide level, however, so evidence for thymineless stress-induced intrachromosomal HR remained only suggestive. Additional evidence for thymineless stress-induced HR may perhaps be inferred from work by Murphy et al [8]. This latter work reports the curious observation that RTX treatment actually increased tumorigenesis in the Apc Min/+ mouse which the investigators were using as an animal model for colon and intestinal cancer.…”

“…Unlike fluoropyrimidines, RTX apparently acts exclusively through TS inhibition. The use of RTX in the treatment of colorectal cancer has been approved in Europe, Canada, Australia and Japan [7,8]. …”

5-Fluorouracil mediated anti-cancer activity in colon cancer cells is through the induction of Adenomatous Polyposis Coli: Implication of the long-patch base excision repair pathway

“…5-FU can also be directly incorporated into RNA and DNA and alter transcription and replication, respectively [15]. Several lines of evidence support a role for APC in 5-FU mediated tumour killing in animal models and samples directly taken from CRC patients [18–20]. In the Apc Min/+ mouse model, 5-FU monotherapy exerts cytostatic effects on intestinal tumour growth, but tumours resume growth upon cessation of treatment [18].…”

“…Several lines of evidence support a role for APC in 5-FU mediated tumour killing in animal models and samples directly taken from CRC patients [18–20]. In the Apc Min/+ mouse model, 5-FU monotherapy exerts cytostatic effects on intestinal tumour growth, but tumours resume growth upon cessation of treatment [18]. A recent report of a mutant Apc rat model noted no effect of 5-FU treatment on tumour number or size [20].…”