Effects of 5-benzylacyclouridine, an inhibitor of uridine phosphorylase, on the pharmacokinetics of uridine in rhesus monkeys: implications for chemotherapy

Pyrimidine metabolism in schistosomes: A comparison with other parasites and the search for potential chemotherapeutic targets

Schistosomes are responsible for the parasitic disease schistosomiasis, an acute and chronic parasitic ailment that affects >240 million people in 70 countries worldwide. It is the second most devastating parasitic disease after malaria. At least 200,000 deaths per year are associated with the disease. In the absence of the availability of vaccines, chemotherapy is the main stay for combating schistosomiasis. The antischistosomal arsenal is currently limited to a single drug, Praziquantel, which is quite effective with a single-day treatment and virtually no host-toxicity. Recently, however, the question of reduced activity of Praziquantel has been raised. Therefore, the search for alternative antischistosomal drugs merits the study of new approaches of chemotherapy. The rational design of a drug is usually based on biochemical and physiological differences between pathogens and host. Pyrimidine metabolism is an excellent target for such studies. Schistosomes, unlike most of the host tissues, require a very active pyrimidine metabolism for the synthesis of DNA and RNA. This is essential for the production of the enormous numbers of eggs deposited daily by the parasite to which the granulomas response precipitates the pathogenesis of schistosomiasis. Furthermore, there are sufficient differences between corresponding enzymes of pyrimidine metabolism from the host and the parasite that can be exploited to design specific inhibitors or "subversive substrates" for the parasitic enzymes. Specificities of pyrimidine transport also diverge significantly between parasites and their mammalian host. This review deals with studies on pyrimidine metabolism in schistosomes and highlights the unique characteristic of this metabolism that could constitute excellent potential targets for the design of safe and effective antischistosomal drugs. In addition, pyrimidine metabolism in schistosomes is compared with that in other parasites where studies on pyrimidine metabolism have been more elaborate, in the hope of providing leads on how to identify likely chemotherapeutic targets which have not been looked at in schistosomes.

Potent combination therapy for human breast tumors with high doses of 5-fluorouracil: remission and lack of host toxicity

PURPOSE

The purpose of this investigation was to evaluate the effectiveness of oral 5-(phenylthio)acyclouridine (PTAU) in reducing 5-fluorouracil (FUra) host toxicity and enhancing its chemotherapeutic efficacy against human breast tumors. PTAU is a potent and specific inhibitor of uridine phosphorylase (UP, EC 2.4.2.3), the enzyme responsible for uridine catabolism.

METHODS

SCID mice bearing MDA-MB-468 and MCF-7 human breast tumors were injected intraperitoneally with FUra (50, 200 or 300 mg/kg) on days 17, 24, and 31 after tumor cell inoculation. PTAU (120 mg/kg), uridine (1,320 mg/kg), or their combination was administered orally two or 4 h after FUra injection. Another four administrations of PTAU plus uridine were given every 8 h after the first treatment with PTAU plus uridine. Survival and body weight were used to evaluate host toxicity. Tumor weight was used to evaluate the efficacy of the drugs on tumor growth. The mice were monitored for 38 days.

RESULTS

Administration of the maximum tolerated dose (50 mg/kg) of 5-fluorouracil (FUra) to SCID mice bearing human breast MDA-MB-468 and MCF-7 adenocarcinoma tumor xenografts reduced tumor weight by 59 and 61%, respectively. Administration of 200 mg/kg FUra resulted in 100% mortality. Oral administration of uridine (1,320 mg/kg) alone, 2 h following the administration of 200 mg/kg FUra, did not rescue from FUra host toxicity as all the mice died. Administration of 120 mg/kg PTAU resulted in partial rescue from this lethal dose of FUra as 38% of inoculated mice survived and the tumor weights were reduced by approximately 67%. Coadministration of PTAU plus uridine resulted in complete rescue from the toxicity of FUra. All of the mice survived, and MDA-MB-468 and MCF-7 tumor weights were reduced by 97% and total remission, respectively. Doubling the FUra treatment dose to 400 mg/kg in the MDA-MB-468 inoculated mice, with the administration of the adjuvant combination treatment of PTAU plus uridine, was unsuccessful in rescuing from FUra toxicity as all the mice died. Lowering the dose of FUra to 300 mg/kg, under the same conditions, resulted in 67% mice survival, and the MCF-7 tumor weights were reduced by 100%. Treatment with uridine alone did not protect from FUra toxicity at 200, 300, and 400 mg/kg as all of the mice died. At the higher dose of 300 and 400 mg/kg FUra, PTAU alone had no rescuing effect. There was no significant difference between MDA-MB-468 and MCF-7 in their response to the different regimens employed in this study in spite of the fact that MDA-MB-468 is estrogen receptor negative while MCF-7 is estrogen receptor positive.

CONCLUSIONS

The present results demonstrate that the combination of PTAU plus uridine represents an exceptionally efficient method in increasing FUra chemotherapeutic efficacy while minimizing its host toxicity. The efficiency of the PTAU plus uridine combination can be attributed to the extraordinary effectiveness of this combination treatment in raising and maintaining higher levels of uridine in vivo (Al Safarjalani et al. in Cancer Chemo Pharmacol 55:541-551, 2005). Therefore, the combination of PTAU plus uridine can provide a better substitute for the massive doses of uridine necessary to rescue or protect from FUra host-toxicities, without the toxic side effects associated with such doses of uridine. The combination may also allow the escalation of FUra doses for better chemotherapeutic efficacy against human breast carcinoma, with the possibility of avoiding FUra host-toxicities. Alternatively, the combination of PTAU and uridine may be useful as an antidote in the few cases when cancer patients receive a lethal overdose of FUra.

Modulation of 5-fluorouracil host-toxicity and chemotherapeutic efficacy against human colon tumors by 5-(Phenylthio)acyclouridine, a uridine phosphorylase inhibitor

PURPOSE

The purpose of this investigation was to evaluate the effectiveness of oral 5-(phenylthio)acyclouridine (PTAU) in reducing 5-fluorouracil (FUra) host-toxicity and enhancing its chemotherapeutic efficacy against human colon tumors. PTAU is a potent and specific inhibitor of uridine phosphorylase (UrdPase, EC 2.4.2.3), the enzyme responsible for uridine catabolism.

METHODS

SCID mice bearing human colon DLD-1 or HCT-15 tumors were injected intraperitoneally with FUra (50, 200 or 300 mg/kg) on days 17, 24 and 31 after tumor cell inoculation. PTAU (120 mg/kg), uridine (1,320 mg/kg) or their combination was administered orally 2 or 4 h after FUra injection. Another four administrations of PTAU+uridine were given every 8 h after the first treatment with PTAU plus uridine. Survival and body weight were used to evaluate host toxicity. Tumor weight was used to evaluate the efficacy of the drugs on tumor growth. The mice were monitored for 38 days.

RESULTS

Administration of the maximum tolerated dose (50 mg/kg) of FUra reduced DLD-1 and HCT-15 tumor weights by 48 and 59%, respectively, at day 38 post implantation. Administration of 200 mg/kg FUra resulted in 100% mortality. Oral administration of uridine (1,320 mg/kg) alone, 2 h following the administration of 200 mg/kg FUra, did not alleviate FUra host-toxicity as all the mice died. Administration of 120 mg/kg PTAUresulted in partial rescue from this lethal dose of FUra as 63% of mice survived and tumor weights were reduced by approximately 60%. Coadministration of PTAU plus uridine resulted in complete rescue from the toxicity of FUra as 100% of the mice survived and tumor weights were reduced by 81-82%. Delaying the administration of the combination of PTAU plus uridine to 4 h post FUra treatment was less effective in rescuing from FUra toxicity as only 88% of the mice survived and tumor weights were reduced by only 62%. Administration of PTAU alone, under the same conditions, resulted in a 38% survival rate while the tumor weights were reduced by 47%. Treatment with uridine alone did not protect from FUra toxicity at the dose of 200 mg/kg as all mice died. At the higher dose of 300 mg/kg FUra, neither uridine nor PTAU alone, administered 2 h following the treatment with FUra, had any rescuing effect. On the other hand, the use of the PTAU plus uridine combination reduced the tumor weight by 79%, although this reduction in the tumor weight was accompanied by 37% mortality. There was no significant difference between DLD-1 and HCT-15 in their response to the different regimens employed in this study despite the fact that the tumors have different levels of UrdPase.

CONCLUSIONS

The present results demonstrate that the combination of PTAU plus uridine represents an exceptionally efficient method in increasing FUra chemotherapeutic efficacy while minimizing its host-toxicity. The efficiency of the PTAU plus uridine combination can be attributed to the extraordinary effectiveness of this combinationin raising and maintaining higher levels of uridine in vivo (Al Safarjalani et al., Cancer Chemo Pharmacol 55:541-551, 2005). Therefore, the combination of PTAU plus uridine can provide a better substitute for the large doses of uridine necessary to rescue or protect from FUra host-toxicities, without the toxic side-effects associated with such doses of uridine. This combination may also allow for the escalation of FUra doses for better chemotherapeutic efficacy against human colon carcinoma while avoiding FUra host-toxicities. Alternatively, the combination of PTAU and uridine may be useful as an antidote in the few cases when cancer patients receive a lethal overdose of FUra.

5-(Phenylthio)acyclouridine: a powerful enhancer of oral uridine bioavailability: relevance to chemotherapy with 5-fluorouracil and other uridine rescue regimens

PURPOSE

The purpose of this investigation was to evaluate the effectiveness of oral 5-(phenylthio)acyclouridine (PTAU) in improving the pharmacokinetics and bioavailability of oral uridine. PTAU is a potent and specific inhibitor of uridine phosphorylase (UrdPase, EC 2.4.2.3), the enzyme responsible for uridine catabolism. This compound was designed as a lipophilic inhibitor in order to facilitate its access to the liver and intestine, the main organs involved in uridine catabolism. PTAU is fully absorbed after oral administration with 100% oral bioavailability.

METHODS

Uridine (330, 660 or 1320 mg/kg) and/or PTAU (30, 45, 60, 120, 240 or 480 mg/kg) were orally administered to mice. The plasma levels of uridine, its catabolite uracil, and PTAU were measured using HPLC, and pharmacokinetic analysis was performed.

RESULTS

Oral PTAU up to 480 mg/kg per day is not toxic to mice. Oral PTAU at 30, 45, 60, 120 and 240 mg/kg has a prolonged plasma half-life of 2-3 h, and peak plasma PTAU concentrations (C(max)) of 41, 51, 74, 126 and 161 microM with AUCs of 70, 99, 122, 173 and 225 micromol h/l, respectively. Coadministration of uridine with PTAU did not have a significant effect on the pharmacokinetic parameters of plasma PTAU at any of the doses tested. Coadministration of PTAU (30, 45, 60 and 120 or 240 mg/kg) with uridine (330, 660 or 1320 mg/kg) elevated the concentration of plasma uridine over that following the same dose of uridine alone, a result of reduced metabolic clearance of uridine as evidenced by decreased plasma exposure (C(max) and AUC) to uracil. Plasma uridine was elevated with the increase of uridine dose at each PTAU dose tested and no plateau was reached. Coadministration of PTAU at 30, 45, 60, 120 and 240 mg/kg improved the low oral bioavailability (7.7%) of uridine administered at 1320 mg/kg by 4.3-, 5.9-, 9.9-, 11.7- and 12.5-fold, respectively, and reduced the AUC of plasma uracil (1227.8 micromol h/l) by 5.7-, 6.8-, 8.2-, 6.3-, and 6.9-fold, respectively. Similar results were observed when PTAU was coadministered with lower doses of uridine. Oral PTAU at 30, 45, 60, 120 and 240 mg/kg improved the oral bioavailability of 330 mg/kg uridine by 1.7-, 2.4-, 2.6-, 5.2- and 4.3- fold, and that of 660 mg/kg uridine by 2.3-, 2.7-, 3.3-, 4.6- and 6.7-fold, respectively.

CONCLUSION

The excellent pharmacokinetic properties of PTAU, and its extraordinary effectiveness in improving the oral bioavailability of uridine, could be useful to rescue or protect from host toxicities of 5-fluorouracil and various chemotherapeutic pyrimidine analogues used in the treatment of cancer and AIDS, as well as in the management of medical disorders that are remedied by the administration of uridine including CNS disorders (e.g. Huntington's disease, bipolar disorder), liver diseases, diabetic neuropathy, cardiac damage, various autoimmune diseases, and transplant rejection.

Rapid determination of rat plasma uridine levels by HPLC-ESI-MS utilizing the Captiva plates for sample preparation

A rapid, accurate and precise HPLC-ESI-MS method for the determination of rat plasma uridine concentrations was developed and is described here. Sample preparation involves methanol precipitation of plasma proteins in a 96-well Captiva protein precipitation filter plate. A clear extract is drawn through the filter plate with vacuum, followed by evaporation of the extract and subsequent reconstitution prior to chromatography on a reversed-phase column with an aqueous mobile phase [0.1% (v/v) glacial acetic acid]. Detection was accomplished by positive-ion electrospray ionization mass spectrometry. A calibration curve ranging in concentration from 0.78 to 25 microM was constructed by best-fit, 1/x weighting linear regression analysis of the calibration standard concentrations vs peak height ratios of analyte with internal standard. The correlation coefficient was >0.995. The overall assay accuracy as shown by the back-calculated concentrations of the calibration curve ranged from 96.6 to 103% with RSD ranging from 4.5 to 20%. While this assay method was developed for the determination of uridine in rat plasma, it could be readily adapted for determination of uridine in plasma from other species, such as human.

5-phenylthioacyclouridine: a potent and specific inhibitor of uridine phosphorylase

5-Phenylthioacyclouridine (PTAU or 1-[(2-hydroxyethoxy)methyl]-5-phenylthiouracil) was synthesized as a highly specific and potent inhibitor of uridine phosphorylase (UrdPase, EC 2.4.2.3). PTAU has inhibition constant (K(is)) values of 248 and 353 nM towards UrdPase from mouse and human livers, respectively. PTAU was neither an inhibitor nor a substrate for thymidine phosphorylase (EC 2.4.2.4), uridine-cytidine kinase (EC 2. 7.1.48), thymidine kinase (EC 2.7.1.21), dihydrouracil dehydrogenase (EC 1.3.1.2), orotate phosphoribosyltransferase (EC 2.4.2.10), or orotidine 5'-monophosphate decarboxylase (EC 4.1.2.23), the enzymes that could utilize the substrate (uridine or thymidine) or products (uracil or thymine) of UrdPase. Different isomers of 5-tolylthiouracil also were synthesized and tested as inhibitors of UrdPase. The meta-substituted isomer was 3- to 4-fold more potent as an inhibitor of UrdPase than the para- or ortho-substituted isomers. These data indicate that the hydrophobic pocket in the active site of UrdPase adjacent to the 5-position of the pyrimidine ring can accommodate the meta-substituted 5-phenyluracils better than the other isomers, leading to improved inhibition. Therefore, it is anticipated that the potency of PTAU can be increased further by the addition of certain hydrophobic groups at the meta position of the phenyl ring. PTAU has potential usefulness in the therapy of cancer and AIDS as well as other pathological and physiological disorders that can be remedied by the administration of uridine.

Modulation of 5-fluorouracil host toxicity by 5-(benzyloxybenzyl)barbituric acid acyclonucleoside, a uridine phosphorylase inhibitor, and 2',3',5'-tri-O-acetyluridine, a prodrug of uridine

Administration of 200 mg/kg of 5-fluorouracil (FUra) to mice bearing human colon carcinoma DLD-1 xenografts resulted in 100% mortality. Oral administration of 2000 mg/kg of 2',3',5'-tri-O-acetyluridine (TAU), a prodrug of uridine, in combination with 120 mg/kg of 5-(benzyloxybenzyl)barbituric acid acyclonucleoside (BBBA), the most potent known inhibitor of uridine phosphorylase (UrdPase, EC 2.4.2. 3), 2 hr after the administration of the same dose of FUra completely protected the mice (100% survival) from the toxicity of FUra. This combination also reduced tumor weight by 67% compared with 46% achieved by the maximum tolerated dose (50 mg/kg) of FUra alone. Similarly, administration of BBBA plus TAU 1 hr before or 4 hr after the administration of FUra reduced the tumor weight by 53 and 37%, respectively. However, these schedules were less effective in protecting the host from the toxicity of FUra than when the treatment was carried out at 2 hr after FUra administration. TAU alone did not protect from FUra host toxicity. The efficiency of the BBBA plus TAU combination in rescuing from FUra host toxicities is attributed to the exceptional effectiveness of this combination in raising and maintaining higher plasma uridine concentrations than those achieved by TAU alone or by equimolar doses of uridine (Ashour et al., Biochem Pharmacol 51: 1601-1612, 1996). The present results suggest that the BBBA plus TAU combination can provide a better substitute for the massive doses of uridine required to achieve the high levels of uridine necessary to rescue or protect from FUra host toxicities without the toxic side-effects associated with such doses of uridine. The combination of TAU plus BBBA may also allow the escalation of FUra doses for better chemotherapeutic efficacy. Alternatively, the combination may be used as a rescue regimen in the occasional cases where cancer patients receive a lethal overdose of FUra.

5-(m-Benzyloxybenzyl)barbituric acid acyclonucleoside, a uridine phosphorylase inhibitor, and 2',3',5'-tri-O-acetyluridine, a prodrug of uridine, as modulators of plasma uridine concentration. Implications for chemotherapy

5-(m-Benzyloxybenzyl)barbituric acid acyclonucleoside (BBBA), the most potent inhibitor known of uridine phosphorylase (UrdPase, EC 2.4.2.3), the enzyme responsible for uridine catabolism, and 2',3',5'-tri-O-acetyluridine (TAU), a prodrug of uridine, were used to investigate the possibility of improving the bioavailability of oral uridine in mice. Oral BBBA administered at 30, 60, 120, and 240 mg/kg increased the concentration of plasma uridine (2.6 +/- 0.7 microM) by 3.2-, 4.6-, 5.4-, and 7.2-fold, respectively. After administration of 120 and 240 mg/kg BBBA, plasma uridine concentration remained 3- and 6-fold, respectively, higher than the plasma concentration at zero time (C0) for over 8 hr. On the other hand, BBBA did not change the concentration of plasma uracil. TAU was far more superior than uridine in improving the bioavailability of plasma uridine. The relative bioavailability of plasma uridine released from oral TAU (53%) was 7-fold higher than that (7.7%) obtained by oral uridine. Oral TAU at 460, 1000, and 2000 mg/kg achieved area under the curve (AUC) values of plasma uridine of 82, 288, and 754 mumol.hr/L, respectively. Coadministration of BBBA with uridine or TAU further improved the bioavailability of plasma uridine resulting from the administration of either alone and reduced the Cmax and AUC of plasma uracil. Coadministration of BBBA at 30, 60, and 120 mg/kg improved the relative bioavailability of uridine released from 2000 mg/kg TAU (53%) by 1.7-, 2.7-, and 3.9-fold, respectively, while coadministration of the same doses of BBBA with an equimolar dose of uridine (1320 mg/kg) increased the relative bioavailability of oral uridine (7.7%) by 4.1-, 5.3-, and 7.8-fold, respectively. Moreover, the AUC and Cmax of plasma uridine after BBBA (120 mg/kg) coadministration with TAU were 3.5- and 11.5-fold, respectively, higher than those obtained from coadministration of BBBA with an equimolar dose of uridine. The exceptional effectiveness of the BBBA plus TAU combination in elevating and sustaining high plasma uridine concentration can be useful in the management of medical disorders that are remedied by administration of uridine as well as to rescue or protect from host-toxicities of various chemotherapeutic pyrimidine analogues.

Selective protection of toxicity of 2',3'-dideoxypyrimidine nucleoside analogs by beta-D-uridine in human granulocyte-macrophage progenitor cells

beta-D-Uridine protected human granulocyte-macrophage lineage cells in both semi-solid (granulocyte-macrophage colony-forming units, CFU-GM) and liquid cultures against the toxic effects of 3'-azido-3'-deoxythymidine (AZT), 3'-fluoro-3'-deoxythymidine (FLT) and a combination of AZT and FLT, without impairment of the activities of these respective drugs against human immunodeficiency virus (HIV) replication. In addition, beta-D-uridine also protected human CFU-GM against toxicity of the in vivo AZT metabolite, 3'-amino-3'-deoxythymidine (AMT). Beta-L-uridine and alpha-D-uridine, two stereoisomers of the natural form, and the base uracil, were unable to protect cells against either AZT or FLT toxicity, whereas beta-D-uridine-5'-bis(SATE)phosphotriester, a prodrug of beta-D-uridine-5'-monophosphate, successfully protected cells against AZT toxic effects, suggesting that beta-D-uridine needs to be metabolized to its nucleotides to exert a pharmacological effect. These data suggest in addition that AZT, FLT and AMT share a common target site(s) of toxicity involved in myelosuppression.