An update on the biochemistry of 5-fluorouracil

Vitamin D potentiates anti-tumor activity of 5-fluorouracil via modulating caspase-3 and TGF-β1 expression in hepatocellular carcinoma-induced in rats

We investigated the role of vitamin D (Vit D) alone and in combination with 5-fluorouracil (5-FU) in thioacetamide (TAA)-induced hepatocellular carcinoma (HCC) in rats. Fifty male Sprague-Dawley rats were randomized into a control group and 4 groups that received TAA (200 mg/kg, i.p.) twice per week for 16 weeks. These 4 groups were further divided as follows: HCC group; 5-FU group (75 mg/kg, i.p., once weekly for 3 weeks starting from the 12th week); Vit D group (200 IU/kg daily by oral tube for 16 weeks); and 5-FU + Vit D group (received the previously mentioned dosage regimens of 5-FU and Vit D). HCC was detected by histopathological changes in liver sections and the elevation of serum α-fetoprotein (AFP). Treatment with 5-FU + Vit D significantly decreased gene expression of nuclear factor erythroid 2-related factor 2 (NrF2) and transforming growth factor β1 (TGF-β1) at both the gene and protein level and serum AFP concentrations in comparison with their corresponding monotherapy. Moreover, 5-FU + Vit D treatment enhanced apoptosis by increasing caspase-3 gene and protein expression. Conclusively, Vit D enhances antitumor activity of 5-FU in an HCC-induced model and improves liver function of treated animals. Combination therapy in a TAA-induced HCC rat model was more effective than 5-FU or Vit D through the modulation of TGF-β1, caspase-3, and NrF2 expressions.

Drug eluting antimicrobial vascular catheters: Progress and promise

Vascular catheters are critical tools in modern healthcare yet present substantial risks of serious bloodstream infections that exact significant health and economic burdens. Drug-eluting antimicrobial vascular catheters have become important tools in preventing catheter-related bloodstream infections and their importance is expected to increase as significant initiatives are expanded to eliminate and make the occurrence of these infections unacceptable. Here we review clinically significant and emerging drug-eluting antimicrobial catheters within the categories of antibiotic, antiseptic, novel bioactive agents and energy-enhanced drug eluting antimicrobial catheters. Important representatives of each category are reviewed from the standpoints of mechanisms of action, physical-chemical properties, safety, in vitro and clinical effectiveness.

Comparative study of the effects of PEGylated interferon-α2a versus 5-fluorouracil on cancer stem cells in a rat model of hepatocellular carcinoma

Cancer stem cells (CSCs) in hepatocellular carcinoma (HCC) possess tumor-initiating, metastatic, and drug resistance properties. This study was conducted to evaluate the effects of PEGylated interferon-α2a (PEG-IFN-α2a) and 5-fluorouracil (5-FU) on the expression of CSC markers and on specific pathways that contribute to the propagation of CSCs in HCC. HCC was initiated in rats using a single intraperitoneal dose of diethylnitrosamine (DENA) (200 mg/kg) and promoted by weekly subcutaneous injections of carbon tetrachloride (CCl4) for 6 weeks. After the appearance of dysplastic nodules, the animals received PEG-IFN-α2a or 5-FU for 8 weeks. CSC markers (OV6, CD90) and molecules related to transforming growth factor β (TGF-β) and other signaling pathways were assessed in hepatic tissues. The PEG-IFN-α2a treatment effectively suppressed the hepatic expression of OV6 and CD90, ameliorated the diminished hepatic expression of TGF-β receptor II (TGF-βRII) and β2-spectrin (β2SP), and significantly reduced the elevated hepatic expression of TGF-β1, interleukin6 (IL6), signal transducer and activator of transcription3 (STAT3), and vascular endothelial growth factor (VEGF). In contrast, the 5-FU treatment failed to reduce the overexpression of CSC markers and barely affected the disrupted TGF-β signaling. Furthermore, it had no effect on angiogenesis or nitrosative stress. PEG-IFN-α2a, but not 5-FU, could reduce the propagation of CSCs during the progression of HCC by upregulating the disrupted TGF-β signaling, suppressing the IL6/STAT3 pathway and reducing angiogenesis.

In vivo effects of immunomodulators in a murine model of Fluorouracil-induced mucositis

BACKGROUND

Fluorouracil (5-FU) is a pyrimidine analogue used as a cancer treatment. Its toxic side effects, including mucositis, are reported to occur in 40% of the treated patients. Because of the inflammatory component of mucositis, we explored the possibility of modulating this condition with an immunomodulatory agent and a tumor necrosis factor-α inhibitor.

OBJECTIVE

The aim of this study was to evaluate the effect of 2 immunosuppressive agents, etanercept and cyclosporine, in a murine model of 5-FU-induced mucositis.

METHODS

To study the short-term effects of 5-FU on mucositis, cyclosporine and etanercept were administered to mice after an injection of 5-FU. The animals (n = 8) were euthanized at 6 hours post-challenge. Hematoxylin and eosin-stained histologic sections of the small intestine were examined for signs of apoptosis. To further examine the potential of cyclosporine in the treatment of 5-FU-induced mucositis in a longer duration, the animals (N = 15) were given 2 challenges of 5-FU within 6 hours. All mice were dosed daily until day 9 with either cyclosporine (100 mg/kg) or phosphate-buffered saline (PBS).

RESULTS

Six hours after 5-FU challenge, 25 mg/kg etanercept and 50 mg/kg cyclosporine had no effect on 5-FU-induced apoptosis (P > 0.05). However, 100 mg/kg cyclosporine significantly reduced the cumulative level of apoptosis >41.6% of the intestinal crypt surface (P < 0.05). During long-term observation, all mice began to lose weight at a rate of approximately 0.8 g/day after 5-FU exposure. The rates of weight loss and survival were not affected by cyclosporine treatment. The diarrhea onset began on day 4 with 46.7% of the PBS-treated mice showing signs of diarrhea compared with 53.3% in the cyclosporine group. The diarrhea score for both groups plateaued on day 7, with a cumulative score of 41 for the PBS group and 50 for the cyclosporine group. Cyclosporine treatment did not affect the diarrhea onset day or severity compared with the PBS-treated group (P > 0.05).

CONCLUSIONS

Our data indicated that etanercept is not a suitable treatment for 5-FU-induced mucositis. Despite decreased apoptosis in the gut, cyclosporine did not affect the severity of the diarrhea or survival. Therefore, we concluded that cyclosporine treatment was only effective in mediating the short-term apoptotic events in the intestines but has no long-term effect on the animals' survival and diarrhea.

The Leishmania donovani UMP synthase is essential for promastigote viability and has an unusual tetrameric structure that exhibits substrate-controlled oligomerization

The final two steps of de novo uridine 5'-monophosphate (UMP) biosynthesis are catalyzed by orotate phosphoribosyltransferase (OPRT) and orotidine 5'-monophosphate decarboxylase (OMPDC). In most prokaryotes and simple eukaryotes these two enzymes are encoded by separate genes, whereas in mammals they are expressed as a bifunctional gene product called UMP synthase (UMPS), with OPRT at the N terminus and OMPDC at the C terminus. Leishmania and some closely related organisms also express a bifunctional enzyme for these two steps, but the domain order is reversed relative to mammalian UMPS. In this work we demonstrate that L. donovani UMPS (LdUMPS) is an essential enzyme in promastigotes and that it is sequestered in the parasite glycosome. We also present the crystal structure of the LdUMPS in complex with its product, UMP. This structure reveals an unusual tetramer with two head to head and two tail to tail interactions, resulting in two dimeric OMPDC and two dimeric OPRT functional domains. In addition, we provide structural and biochemical evidence that oligomerization of LdUMPS is controlled by product binding at the OPRT active site. We propose a model for the assembly of the catalytically relevant LdUMPS tetramer and discuss the implications for the structure of mammalian UMPS.

Hyperammonemia encephalopathy: an important cause of neurological deterioration following chemotherapy

Idiopathic hyperammonemic encephalopathy is an uncommon but frequently fatal complication of chemotherapy. It is characterised by abrupt alteration in mental status with markedly elevated plasma ammonia levels in the absence of obvious liver disease or any other identifiable cause, and frequently results in intractable coma and death. It usually occurs in patients with haematologic malignancies during the period of neutropenia following cytoreductive therapy or bone marrow transplantation, and in solid organ malignancies treated with 5-fluorouracil. Although the aetiology of this syndrome is yet to be determined, it appears to be multi-factorial in nature. Optimal management remains to be formally established, and the critical step is increased awareness of the syndrome by measurement of plasma ammonium levels in patients with neurological symptoms, leading to early diagnosis and the prompt implementation of therapy.

Impact of heterophil granulocyte depletion caused by 5-fluorouracil on infectious bursal disease virus infection in specific pathogen free chickens

The purpose of this study was to investigate the influence of the cytostatic drug, 5-fluorouracil (5-FU), which causes depletion of heterophil granulocytes, on clinical symptoms and histological lesions during the progress of infectious bursal disease virus (IBDV) infection in chickens. The aim was to disclose the mechanism behind the clinical disease symptoms. Three groups of specific pathogen free chickens were used for the experiment. Chickens in groups 1 and 3 were pretreated with 5-FU, while chickens in group 2 were treated with a placebo. After 5 days, the chickens in groups 2 and 3 were inoculated with the classical IBDV strain F52/70. Bursae of Fabricius were sampled at fixed intervals, and the progress of the infection was monitored by various histological techniques and reverse transcriptase-polymerase chain reaction (RT-PCR). We found correlation between histological observations and RT-PCR results. In the 5-FU pretreated chickens, IBDV caused only mild clinical symptoms, even though histological alterations similar to alterations caused by IBDV were still observed. The 5-FU pretreatment resulted in severe heterophil granulocyte depletion by days 2 and 3 after infection (post inoculation) and increased numbers of bursal secretory dendritic cells in the medulla of the follicles. IBDV infection seemed to induce fusion of secretory dendritic cells, resulting in formation of multinucleated giant cells, loaded with apoptotic B cells and virus particles associated with granules of bursal secretory dendritic cells. Our results indicate that the heterophil granulocytes together with the bursal secretory dendritic cells contribute to the outbreak and/or progress of clinical symptoms.

In vivo studies of Gallibacterium anatis infection in chickens

The aim of the present study was to investigate the pathology in normal or immunosuppressed chickens followed intravenous or intraperitoneal inoculation with a well-characterized strain of Gallibacterium anatis. Two groups of 30 15-week-old commercial brown laying chickens were used, having been screened and found negative for Gallibacterium organisms. One group was treated with 5-fluorouracil to promote heterophil depletion, while the other was saline treated. Ten days later 15 chickens from each group were inoculated either intravenously or intraperitoneally with 3.3 x 10(7) colony-forming units of G. anatis strain 12656-12. Subsets of chickens were sacrificed at 3, 12 or 24 h post-infection and examined for lesions. Livers and spleens were examined by culture and by fluorescent in situ hybridization. Intravenously infected birds showed severe septicaemic lesions in both the normal and immunosuppressed birds. Mortality was recorded only in the latter, with an overall rate of 73%. The intraperitoneally infected chickens of normal immune status showed various degrees of localized purulent peritonitis at the inoculation site, but in the immunosuppressed birds the entire peritoneum tended to be involved along with the abdominal organs. This was similar to previous descriptions of natural infections and may represent a useful infection model for detailed analysis of Gallibacterium virulence factors and pathogenesis.

5-fluorouracil selectively inhibits collagen synthesis

BACKGROUND

Fibroproliferative disorders, such as Dupuytren's contracture of the hand, are characterized by excessive production of collagen. 5-Fluorouracil has been used to treat fibroproliferative disorders of the eye and skin and is thought to inhibit thymidylate synthetase blocking DNA replication. 5-Fluorouracil has been shown to down-regulate fibroblast proliferation and differentiation in vitro.

METHODS

This study investigated the dose-dependent effect of 5-fluorouracil on fibroblast extracellular matrix production. Fibroblasts were derived from tendon and primary Dupuytren's disease of the hand, a fibroproliferative disorder of the palmar aponeurosis (n = 4 patients). Total collagen synthesis was determined by means of the incorporation of radiolabeled proline. Fibroblast secretion of the profibrotic factor transforming growth factor-beta1 (TGF-beta1) was measured by a sandwich enzyme-linked immunosorbent assay. Gene expression of collagen types I and III and TGF-beta1 were quantified by means of reverse-transcriptase polymerase chain reaction assays.

RESULTS

The authors found that 5-fluorouracil caused a dose-dependent, selective, and specific decrease in collagen production by Dupuytren's fibroblasts compared with noncollagenous protein synthesis. By contrast, procollagen types I and III mRNA were unaffected by 5-fluorouracil treatment. These changes did not appear to be mediated by alterations in the endogenous secretion of TGF-beta1 or its autocrine effect on collagen metabolism.

CONCLUSIONS

The clinical implication is that 5-fluorouracil could possibly reduce extracellular matrix production and therefore reduce recurrence of Dupuytren's disease of the hand.

Does leucovorin alter the intratumoral pharmacokinetics of 5-fluorouracil (5-FU)? A Southwest Oncology Group study

PURPOSE AND DESIGN

We previously documented that there was an association between the intra-tumoral pharmacokinetics (TPK) of 5-FU and response to therapy with 5-FU and leucovorin (p < .0001). Since we have shown that other modulators of 5-FU, such as methotrexate, interferon and neutrexin alter its TPK, it was of interest to determine if the modulating effect of leucovorin would also alter the tumoral PK of 5-FU. In order to determine the effect of leucovorin on intratumoral 5-FU pharmacokinetics, 23 patients (21 evaluable) underwent 19F magnetic resonance spectroscopy (19F-MRS) twice. The first 19F-MRS was following 5-FU 600 mg/m2 alone, and the second 19F-MRS was following by leucovorin 500 mg/m2 and then 5-FU 600 mg/m2.

RESULTS

A comparison of the intratumoral 5-FU pharmacokinetics indicated that there was no general effect of leucovorin on the intratumoral half-life of 5-FU. In only two of these 21 patients was the half-life of 5-FU altered, and in both cases it was decreased by more than 20%. Partial responses to 5-FU plus leucovorin therapy were seen only in patients with a long intratumoral half-life (trapping) of 5-FU (3 PR in 11 patients with T1/2 > or = 20 minutes, compared to 0 PR in 11 patients with T1/2 < 20 minutes). There was a statistically significant correlation between tumor response and the intratumoral T1/2 of 5-FU, consistent with our prior results in a larger number of patients. However, there was no statistically significant correlation of time-to-progression or survival with classification of the patients into trappers or non-trappers, probably due to the small sample size in this current study.

CONCLUSION

The data reported here are compatible with the hypothesis that leucovorin enhancement of 5-fluorouracil antitumor responses is not mediated by the levels of 5-FU in tumors, but rather, is due to the modulation by leucovorin of cellular metabolic processes that follow the uptake of free 5-FU into the tumor cell. The MRS technique may be useful in selected instances for elucidating the possible metabolic interactions of drugs in vivo.

Co-delivery of an antisense oligonucleotide and 5-fluorouracil using sustained release poly (lactide-co-glycolide) microsphere formulations for potential combination therapy in cancer

Antisense oligonucleotides (AODNs) can selectively inhibit oncogene expression by Watson-Crick hybridisation to target mRNA and are being increasingly considered for use in combination with conventional drugs for potential anticancer therapy. Combination therapy of AODNs and cytotoxic agents using biodegradable polymeric delivery systems potentially offers several advantages including site-specific or organ-directed targeting, protection from digesting enzymes, and improved pharmacokinetics/pharmacodynamics resulting from sustained delivery of the entrapped drugs. Using a model AODN targeting the epidermal growth factor receptor (that is over-expressed in several cancers including breast and brain cancer) and the commonly used cytotoxic agent, 5-fluorouracil (5-FU), we have examined the use of poly (lactide-co-glycolide) (P(LA-GA)) microsphere formulations for co-delivery of these agents. Both agents were either co-entrapped in a single microsphere formulation or individually entrapped in two separate microsphere formulations and release profiles determined in vitro. Using a double emulsion method for preparing the P(LA-GA) microspheres suitable entrapment and sustained release over 35 days was observed in both types of formulation. Release of AODN and 5-FU from all formulations appeared to be biphasic. However, the release rates of the two agents were significantly slower when co-entrapped as a single microsphere formulation compared to those obtained with the separate formulations. Electrophoretic mobility shift assays suggested that this might be, in part, due to an interaction of 5-FU with the oligodeoxynucleotide (ODN). Further, our data suggest that by mixing individual formulations of 5-FU and ODNs at different mass ratios allowed greater flexibility in achieving the desired release profile as well as avoiding potential drug-drug interactions. Thus, co-administration of individual P(LA-GA) microsphere formulations of AODNs and 5-FU, at appropriate mass ratios, appears worthy of further investigation for the potential co-delivery of these anti-cancer agents in vivo.

The effect of UCN-01 (7-hydroxystaurosporine), a potent inhibitor of protein kinase C, on fractionated radiotherapy or daily chemotherapy of a murine fibrosarcoma

PURPOSE

To investigate the effect of UCN-01 (7-hydroxystaurosporine), a potent and selective protein kinase C inhibitor, on fractionated irradiation or daily chemotherapy; cis-diamminedichloroplatinum(II) (cis-DDP) or 5-fluorouracil (5-FU) in vivo. Radiosensitivity and chemosensitivity given in combination with UCN-01 were further studied in vitro to analyze these in vivo results.

METHODS AND MATERIALS

For in vivo studies, single-cell suspension was prepared from fourth generation FSa-II tumors and transplanted subcutaneously into the leg of 8-10-week-old C3Hf/Sed mice. Treatments were initiated when tumors reached an average diameter of 4 mm. Tumor response was studied using tumor growth and growth delay time assays. UCN-01 was given continuously for 7 days using Alzet osmotic pump (4.0 microg/microl/h or approximately 3.2 mg/kg/day). A daily gamma-ray dose of 10 Gy each was given in air for 7 days. Cis-DDP (0.7 mg/kg/day) or 5-FU (20 mg/kg/day) was given by an i.p. injection for 7 days. For in vitro studies, an established FSa-II cell line was used and cell survival was studied by colony formation assay.

RESULTS

UCN-01 acted synergistically with fractionated irradiation, though it was slightly radioprotective in vitro and had no effect on SLD repair. The surviving fraction of the FSa-II cells treated with both UCN-01 and cis-DDP in vitro was lower than the calculated additive effect; however, the sensitizing effect of UCN-01 was not found when combined with either of the chemotherapeutic agents in vivo. Possible causes of synergism of combined UCN-01 and fractionated radiation may be that a continuous UCN-01 treatment inhibited clonogen repopulation during the course of fractionated irradiation and accumulated cells in the G2-M phase where cells are most sensitive to irradiation.

CONCLUSION

UCN-01 is a promising agent that may indirectly interact with fractionated irradiation in vivo but may not with chemotherapeutic agents.

Combination of reduced folates with methotrexate or 5-fluorouracil. Comparison between 5-formyltetrahydrofolate (folinic acid) and 5-methyltetrahydrofolate in vitro activities

Folinic acid (dlFA) is increasingly used in clinical oncology. The active isomer lFA is intensively metabolized into l5-methyltetrahydrofolate (l5MTHF), the relative proportions of lFA, dFA and l5MTHF in blood varying considerably between oral and i.v. FA administration. The purpose of the study was to compare the in vitro activities of pure lFA and pure l5MTHF at equivalent drug exposure [area under curve (AUC)], taking into account their respective chemical stability in the culture medium. The in vitro growth inhibition [3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) test] was evaluated on five human tumor cell lines after methotrexate (MTX)-folate or 5-fluorouracil (5FU)-folate exposures. Not only were the activities of lFA and l5MTHF compared, but also clinically relevant mixtures of lFA + dFA + l5MTHF corresponding to the proportions found at steady state during oral (PO mixture, 4, 39 and 57%, respectively) and i.v. administrations (i.v. mixture, 7, 81 and 12%, respectively). Measurement of folates demonstrated the marked lability of l5MTHF (65.8% loss over 5 days in the culture medium) as compared to lFA (2.6% loss). Whatever the pharmacological model tested (MTX-folate or 5FU-folate), comparison of the folate effects at equivalent drug exposure taking into account their relative stability showed that l5MTHF was never more potent than lFA. Moreover, a higher efficiency of lFA was demonstrated for the cell line most sensitive to 5FU; in this case, as expected, the i.v. mixture was more potent than the PO mixture. This study shows that depending on the tumor, lFA can be more potent than its main circulating metabolite l5MTHF. Along with the limited capacity of oral absorption, the choice between oral and i.v. route for FA administration in patients should take into consideration the different pharmacological activities between lFA and l5MTHF which suggest that the oral route is potentially detrimental to the optimal activity of the 5FU-FA combination as compared to i.v. administration.

Fluorodeoxyuridine with continuous leucovorin infusion. A phase II clinical trial in patients with metastatic colorectal cancer

BACKGROUND

Chemotherapy of colon cancer has used the modulation of 5-fluorouracil (5-FU) by leucovorin; however, early studies indicate that leucovorin with fluorodeoxyuridine (FUDR) may be clinically superior. The authors report their experience in patients with metastatic colorectal cancer.

METHODS

One hundred twelve evaluable patients with metastatic colorectal cancer were treated with leucovorin and FUDR. Leucovorin was given by continuous intravenous infusion at 500 mg/m2/day on days 1-6, and FUDR was given by intravenous push on days 2-6 at 3:00 p.m. daily, with doses ranging from 270-1350 mg/m2/day.

RESULTS

This regimen was well tolerated with dose-limiting toxicity of diarrhea and stomatitis, while hematologic toxicity was minimal. At least one chemotherapy regimen had previously failed in 90 of 112 patients (80%). Twenty major responses greater than or equal to partial remission, lasting from 2-40+ months, were observed in this patient population for an overall response rate of 18%. Twelve of 22 previously untreated patients (55%) had major responses. Overall survival of previously untreated patients was 73% (14 of 19) and 50% (11 of 22) at 1 and 2 years, respectively. Of 66 patients who had received prior leucovorin-5-FU (LVFU) therapy with subsequent disease progression, 4 had major responses lasting 95, 241, 350, and 432 days, respectively.

CONCLUSIONS

These data suggest that the modulation of FUDR by leucovorin may have clinical use. The recommended starting dose of FUDR is 800 mg/m2/day on days 2-6, with subsequent escalation each month in those patients who do not display stomatitis.

The genetic toxicology of 5-fluoropyrimidines and 5-chlorouracil

The halogenated pyrimidines were synthesized in the 1950s as potential anti-tumor agents after the discovery that certain tumors preferentially incorporated uracil rather than thymine into the DNA. The fluorinated derivatives are widely recognized today as effective treatment modalities, especially with tumors of the head, neck and breast. Mechanistically, efficacy of the fluorinated pyrimidines results from the ability of these compounds to incorporate into RNA and inhibit its maturation to those forms necessary for cellular metabolism and from the inhibition of the enzyme, thymidylate synthetase, which controls the biosynthesis of thymine and DNA synthesis. The 5-fluoropyrimidines can incorporate into DNA, but the contribution of this phenomenon to the overall efficacy of this class of chemotherapeutic agents is not totally resolved. Evidence exists that this class of compounds possesses the properties to induce genotoxic effects, both in bacterial and eukaryotic cells. Most notably, these effects include the induction of cellular toxicity and the induction of chromosome aberrations. The biology and chemistry of the chlorinated pyrimidines were first explored as a possible means of sensitizing the DNA to ionizing radiation in a manner similar to the sensitization observed when DNA incorporates bromodeoxyuridine. This approach was not utilized clinically. The genetic toxicology of this compound became important with the discovery of the ribonucleoside in the effluents of sewage treatment plants. Evidence is now available that the chlorinated pyrimidines, upon conversion to deoxyribonucleosides, are effective mutagens, clastogens and toxicants, as well as extremely effective inducers of sister-chromatid exchanges.

Thymidine-modulated 5-fluorouracil metabolism in liver and RIF-1 tumors studied by 19F magnetic resonance spectroscopy

19F Magnetic resonance spectroscopy was used to study the impact of the biochemical modulator thymidine (TdR) on the 5-fluorouracil (5FU) metabolism in the livers and radiation-induced fibrosarcoma (RIF-1) tumors of 5FU-treated C3H mice. The liver spectra measured after administration of 5FU (65 or 130 mg/kg IP) showed the 5 FU resonance and its catabolites alpha-fluoro-beta-ureidopropionic acid and alpha-fluoro-beta-alanine. At the latter dose, fluoronucleotide signal was also detected. The liver spectra of TdR-pretreated (500 mg/kg, IP) mice showed additional signals of fluoronucleotide and fluoronucleoside at both 5FU doses, while alpha-fluoro-beta-alanine was not detected. TdR pretreatment increased the half-life of 5FU in livers from 24 +/- 2 to 126 +/- 46 SEM min at the 5FU dose of 65 mg/kg and from 28 +/- 2 to 95 +/- 22 min at the 130 mg/kg dose (P less than .1 and P less than .01, respectively). TdR-pretreated mice had higher 5FU anabolite (fluoronucleotide + fluoronucleoside) levels in their RIF-1 tumors than nonpretreated mice that received the same 5FU doses (56 +/- 15 SEM vs. 0 arbitrary units at the 5FU dose of 65 mg/kg, and 88 +/- 21 vs. 10 +/- 3 arbitrary units at 130 mg/kg 5FU; P less than .0001). The percentage drop in tumor volume was enhanced in the mice that received TdR, from 27 +/- 4 SEM to 52 +/- 2 at the 5 FU dose of 65 mg/kg and from 24 +/- 3 to 65 +/- 4 at the 130-mg/kg dose (P less than .0001, both).

In vitro enhancement of fluoropyrimidine-induced cytotoxicity by leucovorin in colorectal and gastric carcinoma cell lines but not in non-small-cell lung carcinoma cell lines

Leucovorin (LV) increases the cytotoxic effect of fluorouracil (FUra) and 5-fluoro-2'-deoxyuridine (FdUrd) by enhancing the formation of the fluorodeoxyuridine monophosphate (FdUMP) thymidylate synthase (TS) 5,10-methylenetetrahydrofolate (mTHF) ternary complex. To study the difference in the efficacy of this combination against different tumors, we compared the effect of LV (20 microM) on the cytotoxicity of FUra, FdUrd, and 5-fluorouridine (FUrd) in vitro against cell lines of five colorectal carcinomas (CC), five gastric carcinomas (GC), and four non-small-cell lung carcinomas (NSCLC) using the colony-forming assay. At the concentration used in the experiments, LV alone failed to inhibit colony formation in any of the cell lines tested. The NSCLC cell lines were more resistant to FdUrd than were the CC and GC lines. LV modulated the cytotoxicity of FdUrd in all five CC lines and in three of the five GC lines but failed to do so in any of the NSCLC lines. In addition, following 20 h treatment with 1 microM [3H]-FdUrd, formation of the FdUMP/TS/mTHF ternary complex was enhanced by LV in the LV-sensitized CC and GC cell lines but not in the LV-refractory NSCLC lines. These in vitro data corresponded well to the results of clinical trials. Therefore, the colony-forming assay may be useful for the identification of the sensitivity of tumors according to phenotype.

The effects of 5-fluorouridine, 5-fluorodeoxyuridine, and 5-fluorodeoxyuridine monophosphate on rabbit tenon's capsule fibroblasts in vitro

Inhibition of rabbit subconjunctival fibroblast attachment and proliferation by 5-fluorouridine (FUR), 5-fluoro-2 deoxyuridine (FUdR), and 5-fluoro-2-deoxyuridine-5-monophosphate (FdUMP), was determined by 3H-adenosine uptake, cell counting, and colorimetric assays for the concentration range of 1000 to 0.0001 micrograms/ml over an 9 day period. The mean 50% inhibitory doses against proliferation were calculated for each assay. Rabbit fibroblast attachment was not inhibited at any drug concentration by either FUR, FUdR, or FdUMP. For rabbit fibroblast proliferation, FUR was found to be 10-100 fold more potent than FUdR and FdUMP. When comparing the human and rabbit cells, the unpaired t-test analysis showed no consistent statistical difference of the ID50s for FUR, FUdR or FdUMP. Rabbit ocular fibroblasts may be useful in modeling the proliferation of human ocular fibroblasts. These in vitro results may be useful for predicting optimal drug dosages for future in vivo testing of these drugs.

The effect of 5-fluorouracil at elevated temperatures on a spontaneous mouse tumour: Arrhenius analysis and tumour response

In a series of studies to investigate activation energies and thermal enhancements of various chemotherapeutic agents the effect of 5-fluoruracil (5FU), an antimetabolite, on murine tumour cells was studied at elevated temperatures. Animal tumours were early generation isotransplants of a spontaneous fibrosarcoma, FSa-II tumours, and C3Hf/Sed mice were used throughout. Cell survival curves for 5FU were obtained as a function of treatment time by in vitro treatment-in vivo lung colony assay at temperatures between 37.0 and 43.5 degrees C. The D0, or the treatment time to reduce surviving fraction from S to S/e in the exponential portion of the survival curve, decreased slightly from 37 degrees C treatment to 41.5 degrees C treatment. The D0 decreased substantially from 41.5 degrees C to 43.5 degrees C. Arrhenius-plot analysis indicated that the activation energies were 92.9 and 731 kJ/M at temperatures between 37 and 41.5 degrees C, and between 41.5 and 43.5 degrees C, respectively. The activation energy of 92.9 kJ/M for the temperature range from 37 to 41.5 degrees C was the lowest of the other agents, bleomycin, cis-diamminedichloroplatinum and 1,3-bis(2-chloroethyl)-N-nitrosourea, which have been investigated in our laboratory. This indicated that the thermal enhancement was smallest among these agents. In vivo experiments failed to demonstrate thermal enhancement of the anti-tumour effect of 5FU. Namely, combined 5FU and heat treatments at 41.5 and 43.5 degrees C did not prolong the tumour growth time compared with 5FU given at room temperature. No pH effect was found in an in vitro experiment and glucose administration did not enhance the anti-tumour effect of 5FU.

Antitumor activity of FTC-092, a masked 5-trifluoromethyl-2'-deoxyuridine derivative

1-(3-O-Benzyl-2-deoxy-beta-D-ribofuranosyl)-5-trifluoromethyl-2,4(1H,3)- pyrimidinedione (FTC-092), a fluorinated pyrimidine derivative, appeared to be effective against various transplantable tumors in mice following oral administration, and its activity was superior to that of several other antitumor fluorinated pyrimidines. The ED50 value for FTC-092 the dose effective in achieving 50% inhibition of tumor growth against the solid form of sarcoma 180 was 13.3 mg/kg daily, whereas those for 5-trifluoromethyl-2'-deoxyuridine (CF3dUrd), the parent compound of FTC-092, for 1-(2-tetrahydrofuryl)-5-fluorouracil (Tegafur, FT), the prodrug of 5-fluorouracil (FUra), and for FUra were 64.1, 122, and 28 mg/kg daily, respectively. The therapeutic indices (LD10/ED50) of FTC-092, CF3dUrd, FT, and FUra were 4.39, 1.7, 1.35, and 1.65, respectively. FTC-092 itself is not an active agent. After it has been absorbed from the gastrointestinal tract, FTC-092 undergoes a gradual biotransformation, mainly via the action of liver microsomes, releasing CF3dUrd over a long period. The levels of CF3dUrd in the stomach and small intestine of mice after the oral administration of FTC-092 were undetectable, whereas those following the administration of CF3dUrd at the same dose were high for a period of several hours. In contrast, the CF3dUrd level generated in plasma after the administration of FTC-092 remained at a high level for a longer period than did that observed on the administration of CF3dUrd. The low levels of CF3dUrd measured in stomach and small-intestine tissues and the maintenance of CF3dUrd in blood over long periods after the administration of FTC-092 are features that favor the possible clinical application of FTC-092.

Corneal penetration of 5-fluorouracil and its improvement by prodrug derivatization in the albino rabbit: implication in glaucoma filtration surgery

The objective of this study was to determine whether the corneal penetration of 5-fluorouracil (5-FU) could be altered by prodrugs. The prodrugs studied included various 1-alkoxy-carbonyl 5-FU derivatives as well as an 1-acyloxymethyl and an 3-acyl derivative. Corneal penetration of 5-FU and its prodrugs was evaluated over 180 min using the isolated albino rabbit cornea in the modified Ussing chamber. 5-FU and its prodrugs were analyzed by reversed phase HPLC. Corneal penetration of 5-FU was very poor due to its low lipophilicity and extensive metabolism during penetration. There was no conclusive evidence for carrier-mediated transport. The corneal epithelium offered the main diffusional and metabolic resistance. Prodrugs of 5-FU with improved lipophilicity dramatically increased the corneal penetration of 5-FU, suggesting that a much reduced dose of 5-FU could be used if corneal penetration and aqueous levels are necessary. It is anticipated that a similar lower dose of prodrug would be required should the prodrug also improve the diffusion of 5-FU across the sclera following subconjunctival injection. Whether dose reduction will lead to reduced corneal toxicity in glaucoma filtration surgery following topical or subconjunctival dosing is an interesting therapeutic opportunity that remains to be determined. In addition to enhancing 5-FU penetration, prodrugs also protected 5-FU from metabolism even though they were rapidly and quantitatively converted to 5-FU during penetration. Protection of 5-FU from metabolism in the cornea may be advantageous from the standpoint of sparing the cornea from toxicity caused by 5-FU, be it from topical or subconjunctival dosing.

Modulating effect of alpha-[N,N-[bis(2-hydroxyethyl)]-amino-N- (o-methoxyphenyl)-pyrrolidin-2,5-dione on the chemotherapy of murine tumours

alpha-[N,N-[bis(2-hydroxyethyl)]-amino]-N- (o-methoxyphenyl)pyrrolidin-2,5-dione (I), an intermediate in the synthesis of pyrrolidinedione-N-mustards, did not exhibit antitumour activity against P388 lymphocytic leukemia, Sarcoma 180 (ascites) and Ehrlich (ascites) carcinoma tumours. The effect of co-administration of (I) with established anticancer drugs was studied against these murine tumours. The activity of 5-fluorouracil against Sarcoma 180 (ascites) and Ehrlich (ascites) carcinoma was significantly enhanced by co-administration with (I). Other anticancer drugs, when co-administered with (I), did not show any enhancing effect.

Effect of 5-fluorouracil combination therapy on RNA processing in human colonic carcinoma cells

We have evaluated the RNA-directed cytotoxicity of 5-fluorouracil (5-FU) in human colonic carcinoma cells. The mode of action of 5-FU and its effects on human pre-rRNA processing were then examined. From these data, possible reasons why the disruption of pre-rRNA maturation could induce cytotoxic effects are considered. The results imply that inhibition of thymidylate synthase is not the sole primary cytotoxic lesion in this cell line. First, exogenous thymidine (dTHd) enchanced cytotoxicity. Second, addition of dThd to the cells was found to enhance incorporation of 5-FU into total cellular RNA. Third, 5-FU disrupted rRNA processing by a different mechanism from actinomycin D and methotrexate (MTX), suggesting that the inhibition was not just a consequence of cell death. Finally, the addition of dThd was found to enhance the disruption of rRNA processing consistent with an increase in concentration of 5-FU. These data are discussed in the light of literature reports and their potential for optimising 5-FU protocols.

Metabolism of pyrimidine analogues and their nucleosides

The pyrimidine antimetabolite drugs consist of base and nucleoside analogues of the naturally occurring pyrimidines uracil, thymine and cytosine. As is typical of antimetabolites, these drugs have a strong structural similarity to endogenous nucleic acid precursors. The structural differences are usually substitutions at one of the carbons in the pyrimidine ring itself or substitutions at on of the hydrogens attached to the ring of the pyrimidine or sugar (ribose or deoxyribose). Despite the differences noted above, these analogues, can still be taken up into cells and then metabolized via anabolic or catabolic pathways used by endogenous pyrimidines. Cytotoxicity results when the antimetabolite either is incorporated in place of the naturally occurring pyrimidine metabolite into a key molecule (such as RNA or DNA) or competes with the naturally occurring pyrimidine metabolite for a critical enzyme. There are four pyrimidine antimetabolites that are currently used extensively in clinical oncology. These include the fluoropyrimidines fluorouracil and fluorodeoxyuridine, and the cytosine analogues, cytosine arabinoside and azacytidine.

Metabolism and mechanism of action of 5-fluorouracil

This is a review on the mechanism of action of FUra. Three main areas are addressed: metabolism, RNA-directed actions of FUra, and DNA-directed actions of FUra. Key words for bibliographic purposes: metabolism, RNA, rRNA, mRNA, tRNA, DNA primase, DNA, thymidylate synthetase, uracil N-glycosylase, FUra, FUrd, FdUrd, FdUMP, RNA splicing, 5,10-methylene tetrahydrofolate, FUTP.

A reversible selection system for UMP synthase gene amplification and deamplification

The bifunctional enzyme UMP synthase provides a unique reversible selection system whereby cells that have amplified the UMP synthase gene can be isolated from a wild-type population and cells that have deleted the extra genes can be selected from a population with amplified copies of the gene. UMP synthase catalyzes the conversion of orotic acid to orotidine 5'-monophosphate (OMP) and then OMP to UMP. In the amplification step, Chinese hamster lung cells are selected for resistance to pyrazofurin and 6-azauridine, two inhibitors of the orotidine 5'-decarboxylase activity that converts OMP to UMP. The resistant cells have increased levels of both activities of UMP synthase as a result of a stable amplification of the UMP synthase gene. The deamplification step depends on 5-fluorouracil (5FU), which is converted to its monophosphate form by the orotate phosphoribosyltransferase activity of UMP synthase. Thus cells with increases in this activity are more sensitive to 5FU cytotoxicity, permitting single-step selection of revertants that have lost their amplified UMP synthase genes. These 5FU-selected cells are similar to the parental cell line in their level of UMP synthase activity and number of UMP synthase gene copies. Reselection in increasing concentrations of pyrazofurin and 6-azauridine allows one to isolate cells that have reamplified the UMP synthase gene. The ability to cycle cells of a single lineage through states of amplification and deamplification will facilitate study of the gene amplification process and the factors that influence the composition and stability of amplified regions.

Antitumor activity of BOF-A2, a new 5-fluorouracil derivative

A compound containing both CNDP (3-cyano-2,6-dihydroxypyridine), an inhibitor of 5-fluorouracil (5-FU) degradation, and EM-FU (1-ethoxymethyl-5-fluorouracil), a masked form of 5-FU, was synthesized and named BOF-A2 (3-[3-(6-benzoyloxy-3-cyano-2-pyridyloxycarbonyl)benzoyl]-1-ethoxy methyl-5- fluorouracil). The antitumor activity of BOF-A2 was investigated in sarcoma-180-bearing mice and Yoshida sarcoma-bearing rats. The ED50 (the dose for 50% inhibition) values of BOF-A2 were 25 mg/kg against sarcoma-180 and 15 mg/kg against Yoshida sarcoma. In vitro studies showed that BOF-A2 was rapidly degraded to EM-FU and CNDP in homogenates of the liver and small intestine of mice and rats, and in sera of mice, rats and human, and the conversion of EM-FU to 5-FU occurred only in the microsomal fraction of rat liver in the presence of NADPH. After oral administration of BOF-A2 at 15 mg/kg to Yoshida sarcoma-bearing rats, BOF-A2 was hydrolyzed to EM-FU, CNDP and 5-FU, and their maximum concentrations in the blood were 2000 ng/ml, 300 ng/ml and 40 ng/ml, respectively. Moreover when BOF-A2 was given at the same dose to tumor-bearing mice and rats, the 5-FU levels in the tumor tissue increased much more than those in the blood and persisted for more than 8 h, whereas those in the blood decreased more rapidly. This accumulation and maintenance of a high level of 5-FU in the tumor tissue are concluded to be related to the high antitumor activity of BOF-A2.

Interaction of deoxyuridine with fluorouracil and dipyridamole in a human colon cancer cell line

We have reported previously that dipyridamole increases the toxicity of 5-fluorouracil and alters fluorouracil metabolism in HCT 116 cells, producing a selective increase in fluorodeoxyuridine monophosphate (FdUMP) levels by blocking the efflux of fluorodeoxyuridine. Dipyridamole also blocks deoxyuridine efflux and prolongs the intracellular half-life of deoxyuridine monophosphate (dUMP). The significance of the effect of dipyridamole on FdUMP and dUMP levels was explored further. In cell growth experiments, 1-50 microM deoxyuridine enhanced the cytotoxicity of 5 microM fluorouracil in a dose-dependent manner, and greater than or equal to 10 microM deoxyuridine increased the augmentation of fluorouracil toxicity produced by 0.5 microM dipyridamole. The effect of deoxyuridine on [6-3H]fluorouracil metabolism was studied. After 4 hr, 25 microM deoxyuridine increased the amount of [3H]FdUMP formed 2- to 4-fold relative to that of fluorouracil +/- dipyridamole alone. The mechanism by which deoxyuridine increased FdUMP was examined by measuring the distribution of [2'-3H]deoxyuridine metabolites following exposure of 25 microM deoxyuridine +/- 5 microM fluorouracil. Tritium appeared in the FdUMP peak at 4 and 24 hr in cells exposed to fluorouracil and deoxyuridine, indicating that [3H]deoxyribose was transferred to fluorouracil. A large buildup of [3H]dUMP was seen in cells exposed to fluorouracil plus deoxyuridine for 4 and 24 hr compared to exposure to [3H]deoxyuridine alone, suggesting that dUMP may also inhibit catabolism of FdUMP. Since the increased FdUMP levels produced by dipyridamole did not appear to correlate with further depletion of thymidine triphosphate pools, the incorporation of [3H]fluorouracil metabolites into nucleic acids was monitored by cesium sulfate density centrifugation. Fluorouracil-RNA increased as a function of time (1, 2 and 13 pmol/10(6) cells after 4, 8 and 24 hr), but fluorouracil-DNA was detected only after 24 hr (0.5 pmol/10(6) cells). Dipyridamole however, did not appear to alter the pattern of incorporation of fluorouracil into either RNA or DNA. Perturbations of endogenous dUMP levels by fluorouracil and dipyridamole were then studied. In cells exposed to fluorouracil alone, dUMP pools were unchanged from control at 2 hr, but they had increased 9-fold by 4 hr (3362 pmol/10(6) cells). Simultaneous exposure to fluorouracil and dipyridamole resulted in a 1.5-fold (566 pmol/10(6) cells) and 13.6-fold (5049 pmol/10(6) cells) increase over control dUMP levels after 2 and 4 hr respectively. The dUMP pools continued to enlarge through 24 hr. The effect of fluorouracil on DNA fragility was examined.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhancement of 5-fluorouracil's anticancer activity by dipyridamole

Although the interaction between FUra and DP in HCT 116 cells is fairly complex, data from other investigators indicate that in cell lines in which inhibition of TS is growth limiting at relatively low concentrations of fluoropyrimidines, DP appears to augment the cytotoxicity of FUra and FdUrd by blocking the salvage of dThd (Miller et al., 1987; Schwartz et al., 1987). The previous in vitro data regarding the ability of DP to modulate the toxicity of fluoropyrimidines was obtained in exponentially growing cells. An additional observation that warrants consideration is a report that the inhibition of nucleoside incorporation by DP changed as a function of time in culture (Zhen et al., 1986). Hepatoma 3924A cells in lag and log phase were highly sensitive to DP with IC50 values for dThd incorporation of 0.2 and 0.32 microM, respectively. In contrast, stationary phase cells were insensitive to DP (IC50 = 38.9 microM). Amphotericin B, an antifungal agent which perturbs cell membranes, restored the sensitivity to DP in stationary cells. Several investigators have presented information on the effect of DP on fluoropyrimidines in normal tissues. Lee and Park (1987) examined the effect of DP on FUra and MTX toxicity in a soft-agar cloning assay against two human cancer cell lines and on pooled normal human bone marrow (CFU-C). DP (1 microM) potentiated the action of both MTX (0.1 microM) and FUra (5 microM) on Hep-2 (epidermoid carcinoma), MCF-7 (breast carcinoma) and CFU-C in medium supplemented with either non-dialyzed or dialyzed serum. Woodcock et al. (1987) incubated gallbladder mucosa, obtained from patients undergoing elective surgery for cholelithiasis, with control medium or varying concentrations of DP for 1 hr, and then exposed the mucosal cells to 2.5 microCi [3H]-FdUrd (2.5 microM). After 1 hr, the uptake of FdUrd into the tissue was inhibited to 49% and 42% of control by 0.1 microM and 1 microM, respectively.

Induction of SOS responses in Escherichia coli by 5-fluorouracil

The inducibility of SOS responses by 5-fluorouracil (5-FU), which has been used as an antitumor drug, was studied in Escherichia coli cells which have different DNA repair capacities for UV lesions. Expression of the umuC gene was apparently induced by 5-FU in the wild-type and uvrA strains, but not in lexA and recA strains. The inducibility of the umuC gene by 5-FU, the metabolite of which inhibits thymidylate synthetase, was abolished in cultures containing deoxythymidine monophosphate which is converted from deoxyuridine monophosphate by thymidylate synthetase. These results suggest that 5-FU may exert its SOS inducibility by inhibiting thymidylate synthetase and then disturbing DNA metabolism but not by incorporating 5-FU residues into RNA. Further, 5-FU weakly induced mutations in E. coli.

Therapy of malignant brain tumors: comparison of the in vitro activities of vidarabin-monophosphate, BCNU and 5-fluorouracil

BCNU (carmustine), 5-Fluorouracil (5-FU) and Vidarabin-monophosphate (ARA-A5'P) were compared in their activities against 30 cell lines of primary (n = 21) and metastatic (n = 9) brain tumors, which were characterized in tissue culture by cytochemical, immunological and cytogenetic criteria. In vivo achievable concentration-time products were correlated with in vitro pharmacokinetic data. A micro assay was employed to screen for drug toxicity in individual tumor cell lines; cells were exposed to the drugs at exposure doses relevant to in vivo pharmacokinetics. After 5-8 population doubling times of untreated controls, RNA-synthesis, as a parameter of cell metabolism and proliferation, was determined by incorporation of (5, 6-3H)-uridine into cellular RNA (liquid scintillation counting protocol). A tumor stem cell assay was performed under similar conditions. The cytotoxic effect of each drug on individual cell lines was expressed in terms of a sensitivity index SI (SI = 1 indicating complete resistance) to compare effects of different drugs on the individual tumor cell lines. Mean sensitivity indices for ARA-A5'P, BCNU and 5-FU in brain tumor cell lines (in brackets: primary CNS-tumors) were 0.64 (0.59), 0.89 (0.82) and 0.35 (0.33) respectively. 5-FU was significantly more active than BCNU and ARA-A5'P (P less than 0.001), whereas BCNU was significantly less active than ARA-A5'P (P less than 0.001). ARA-A5'P had a suppressive effect on formation of brain tumor stem cell colonies. There was no cross-resistance of ARA-A5'P to either BCNU or 5-FU. We conclude that ARA-A5'P and 5-FU are potent agents in experimental therapy of human brain tumors, compared with BCNU.

Structure-activity relationship of pyrimidine base analogs as ligands of orotate phosphoribosyltransferase

Eighty pyrimidine base analogs were evaluated as inhibitors of mouse liver orotate phosphoribosyltransferase (OPRTase, EC 2.4.2.10). Based on these findings and an extensive literature review, a structure-activity relationship has been formulated for the binding of pyrimidine base analogs to OPRTase. This study provides a basis for the rational design of new inhibitors of this enzyme, and several such compounds are proposed. Additionally, 4,6-dihydroxypyrimidine has been found to be a potent OPRTase inhibitor. Eleven OPRTase inhibitors were also evaluated as inhibitors of orotidine 5'-monophosphate decarboxylase (ODCase, EC 4.1.2.23). 5-Azauracil, 5-azaorotate, and barbituric acid inhibited ODCase significantly only after preincubation with PRPP and MgCl2 in the presence of cytosol.

Modulation of 5-fluorouracil metabolism by thymidine. In vivo and in vitro studies on RNA-directed effects in rat liver and hepatoma

The effects of thymidine (TdR) co-administration on the cytotoxicity and incorporation of 5-fluorouracil (5-FU) into RNA of various tissues was studied in rats bearing an ascites hepatoma (AH 130). The role of pyrimidine degradation in determining the modulating effects of TdR on the formation of FU-RNA was studied in hepatocytes and AH 130 cells in vitro. TdR (500 mg/kg) potentiated the antitumour effect of 5-FU (150 mg/kg) and also increased host toxicity as judged by changes in body weight. TdR given alone did not significantly affect tumour growth and body weight gain. Examination of the effect of TdR on the incorporation of 5-FU into RNA revealed a differential modulation of RNA-directed toxicity in different tissues. Incorporation of 5-FU into RNA in tumour and bone marrow was increased 2- and 4-fold, respectively. In spleen and kidney the incorporation increased by approximately 50%, but the values did not reach statistical significance. In contrast, the incorporation into RNA of liver and intestinal mucosa was decreased to ca 35% of the control. TdR at concentrations of 40 microM-40 mM progressively inhibited the degradation of 5-FU and decreased the incorporation of 5-FU into RNA of hepatocytes in vitro. In AH 130 cells in vitro TdR did not significantly influence the metabolism of 5-FU and the incorporation into RNA. These results demonstrate that the enhanced incorporation of 5-FU into tumour RNA in vivo after pretreatment with TdR is related not to local effects on the tumour cells but rather to an increased bioavailability of the drug. Although co-administration of TdR did not selectively enhance the antitumour effect of 5-FU, a differential toxicity in host tissues was indicated by the modulated incorporation of 5-FU into RNA.

A gas chromatographic assay for the determination of 5,6-dihydrofluorouracil and 5-fluorouracil in human plasma

A gas chromatographic assay for the determination of 5-fluorouracil (5-FU) and 5,6-dihydrofluorouracil (FDHU) is described. The selectivity and sensitivity of the method allows the determination of both 5-FU and FDHU in 200 microliters of plasma. Diphenylsuccinimide and chlorouracil were used as external and internal standard, respectively. The assay including the extraction shows a good linearity in the range 0-5000 ng/ml plasma for 5-FU as well as for FDHU. 5-FU and FDHU plasma concentrations of a number of patients with breast cancer treated with 5-FU were determined in order to demonstrate the usefulness of the method.

Enhancement and depression by inosine of the growth inhibitory action of 5-fluorouracil on cultured Jensen tumor cells

Continuous exposure of Jensen tumor cells in vitro to 1 mM inosine following a 1 hr exposure to 5-fluorouracil resulted in a 5-fold potentiation of growth inhibition. This effect was abolished by the simultaneous presence of 1 mM cytidine and was attributable to altered metabolic processing of drug anabolites after the uptake of 5-fluorouracil had ceased. In contrast, antagonism to 5-fluorouracil was seen when the cells were exposed successively to 1 mM inosine for 1.5 hr and to 5-fluorouracil for 1 hr. In this case the inhibitory action of the drug was diminished by nearly one-half. Quantitation of the potentiation and antagonism was based upon growth delays measured from growth curves obtained by serial photomicrography.

Dose-dependent metabolism, therapeutic effect, and toxicity of anticancer drugs in man

Dose-dependent metabolism is seen for a number of anticancer drugs, and they provide examples of several different types of dose-dependent metabolic processes. Arabinosyl cytosine and 5-fluorouracil are drugs whose catabolism is saturated at high doses. Therapeutic response to both drugs has been linked to plasma concentration of parent compound, and a nonlinear dose-response relationships might exist at high doses. L-Alanosine also appears to exhibit saturable metabolism at high doses, and this might be responsible for the rapid onset of L-alanosine toxicity as the dose is increased. Isophosphamide is a drug which requires metabolic activation to exert its biological effect, and saturation of metabolism seen at high dose could lead to a plateau in the dose-response relationship. Thymidine exhibits saturation of metabolism at high doses due in part to product inhibition of metabolism. Product inhibition of metabolism has also been suggested for arabinosyl cytosine. Plasma elimination of thymidine and possible elimination of 6-diazo-5-oxo-L-norleucine exhibit characteristics of a dose-dependent "memory" effect. Dose-dependent metabolism of methotrexate is unusual in that formation of the presumed toxic metabolite increases with increase in dose and is associated with a qualitative change in the pattern of drug toxicity at high compared to low doses of drug. The relationship between dose and toxicity of drugs and other foreign compounds is poorly understood in man. Anticancer drugs are one of the few classes of compounds where the relationship of toxicity to dose is reasonably well documented in human subjects. Saturation of metabolism leading to dose-dependent pharmacokinetics occurs with several anticancer drugs, and different types of dose-dependent metabolism have been covered in this review. An attempt has been made to show how dose-dependent metabolism of anticancer drugs might relate to their toxicity. Principles of dose-dependent toxicity seen with anticancer drugs might usefully be applied to other classes of compounds, particularly compounds exhibiting cytotoxicity but also other forms of toxicity. Dose-dependent metabolism could also be important in determining the therapeutic effect of anticancer drugs, and application of principles of dose-dependent metabolism to the development of new chemotherapeutic regimens migh lead to more effective cancer chemotherapy.