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

Prompt treatment with uridine triacetate improves survival and reduces toxicity due to fluorouracil and capecitabine overdose or dihydropyrimidine dehydrogenase deficiency

Uridine triacetate has been shown to be an effective antidote against mortality and toxicity caused by either overdoses or exaggerated susceptibility to the widely used anticancer agents 5-fluorouracil (5-FU) and capecitabine. However, a direct assessment of efficacy based on when emergency treatment was initiated was not clinically feasible. In this study we used mouse models of 5-FU overdose and of dihydropyrimidine dehydrogenase (DPD) deficiency to compare the efficacy of uridine triacetate in reducing toxicity and mortality when treatment was initiated at time points from 4 to 144 h after administration of 5-FU. We found that uridine triacetate was effective both in the 5-FU overdose and DPD deficiency models. Starting treatment within 24 h was most effective at reducing toxicity and mortality in both models, while treatment starting more than 96 to 120 h after 5-FU was far less effective. Uridine triacetate also reduced mortality in the DPD deficiency model when mice were treated with the 5-FU prodrug capecitabine. The results of this study are supportive of clinical observations and practice, indicating that efficacy declined progressively with later and later treatment initiation. Prompt treatment with uridine triacetate, within 24 h, conferred the greatest protection against 5-FU overexposure.

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.

Use of uridine triacetate for the management of fluorouracil overdose

PURPOSE

The use of uridine triacetate for the management of fluorouracil toxicity is reported.

SUMMARY

A 55-year-old man with malignant neoplasm of the sigmoid colon (stage IIIC) was seen in an outpatient chemotherapy center for his first six-month regimen of leucovorin calcium, fluorouracil, and oxaliplatin. Fluorouracil 2400 mg/m(2) i.v. was prescribed to be given over the next 46 hours at a home infusion center. Due to a medication error, a home infusion pharmacist incorrectly programmed the 46-hour infusion of fluorouracil to be administered over 4 hours. To manage the fluorouracil overdose, the physician decided to start the patient on uridine triacetate. The patient received his first dose of uridine triacetate 18 hours after the fluorouracil overdose. He was admitted to the hospital for observation and daily laboratory tests during treatment with uridine triacetate. He received ondansetron (as the hydrochloride salt) 8 mg orally 20 minutes before each dose of uridine triacetate to prevent nausea and vomiting. Uridine triacetate 11 g every 6 hours was administered orally for a total of 20 doses. It was mixed with applesauce at the time of administration and followed with 8 oz of water. The patient's laboratory values remained stable. The patient did not experience any nausea or vomiting during treatment. He was discharged from the hospital on day 5, with no clinical complications and an Eastern Cooperative Oncology Group Performance score of 0.

CONCLUSION

A patient with colon cancer who had received an overdose of fluorouracil was successfully treated with a five-day course of oral uridine triacetate.

Enhanced uridine bioavailability following administration of a triacetyluridine-rich nutritional supplement

Background

Uridine is a therapy for hereditary orotic aciduria and is being investigated in other disorders caused by mitochondrial dysfunction, including toxicities resulting from treatment with nucleoside reverse transcriptase inhibitors in HIV. Historically, the use of uridine as a therapeutic agent has been limited by poor bioavailability. A food supplement containing nucleosides, NucleomaxX®, has been reported to raise plasma uridine to supraphysiologic levels.

Methodology/Principal Findings

Single- and multi-dose PK studies following NucleomaxX® were compared to single-dose PK studies of equimolar doses of pure uridine in healthy human volunteers. Product analysis documented that more than 90% of the nucleoside component of NucleomaxX® is in the form of triacetyluridine (TAU). Single and repeated dosing with NucleomaxX® resulted in peak plasma uridine concentrations 1–2 hours later of 150.9±39.3 µM and 161.4±31.5 µM, respectively, levels known to ameliorate mitochondrial toxicity in vitro. C_max and AUC were four-fold higher after a single dose of NucleomaxX® than after uridine. No adverse effects of either treatment were observed.

Conclusions/Significance

NucleomaxX®, containing predominantly TAU, has significantly greater bioavailability than pure uridine in human subjects and may be useful in the management of mitochondrial toxicity.

Plasma pharmacokinetics and oral bioavailability of the 3,4,5,6-tetrahydrouridine (THU) prodrug, triacetyl-THU (taTHU), in mice

PURPOSE

Cytidine drugs, such as gemcitabine, undergo rapid catabolism and inactivation by cytidine deaminase (CD). 3,4,5,6-tetrahydrouridine (THU), a potent CD inhibitor, has been applied preclinically and clinically as a modulator of cytidine analogue metabolism. However, THU is only 20% orally bioavailable, which limits its preclinical evaluation and clinical use. Therefore, we characterized THU pharmacokinetics after the administration to mice of the more lipophilic pro-drug triacetyl-THU (taTHU).

METHODS

Mice were dosed with 150 mg/kg taTHU i.v. or p.o. Plasma and urine THU concentrations were quantitated with a validated LC-MS/MS assay. Plasma and urine pharmacokinetic parameters were calculated non-compartmentally and compartmentally.

RESULTS

taTHU did not inhibit CD. THU, after 150 mg/kg taTHU i.v., had a 235-min terminal half-life and produced plasma THU concentrations >1 μg/mL, the concentration shown to inhibit CD, for 10 h. Renal excretion accounted for 40-55% of the i.v. taTHU dose, 6-12% of the p.o. taTHU dose. A two-compartment model of taTHU generating THU fitted the i.v. taTHU data best. taTHU, at 150 mg/kg p.o., produced a concentration versus time profile with a plateau of approximately 10 μg/mL from 0.5-2 h, followed by a decline with a 122-min half-life. Approximately 68% of i.v. taTHU is converted to THU. Approximately 30% of p.o. taTHU reaches the systemic circulation as THU.

CONCLUSIONS

The availability of THU after p.o. taTHU is 30%, when compared to the 20% achieved with p.o. THU. These data will support the clinical studies of taTHU.

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.

6-Benzylthioinosine analogues as subversive substrate of Toxoplasma gondii adenosine kinase: activities and selective toxicities

Toxoplasma gondii adenosine kinase (EC.2.7.1.20) is the major route of adenosine metabolism in this parasite. The enzyme is significantly more active than any other enzyme of the purine salvage in T. gondii and has been established as a potential chemotherapeutic target for the treatment of toxoplasmosis. Certain 6-substituted purine nucleosides act as subversive substrates of T. gondii, but not the human, adenosine kinase. Therefore, these compounds are preferentially metabolized to their respective nucleotides and become selectively toxic against the parasites but not their host. Herein, we report the testing of newly synthesized 6-benzylthioinosine analogues with various substituents on the phenyl ring of their benzyl group as subversive substrates of T. gondii adenosine kinases. The binding affinity of these compounds to T. gondii adenosine kinase and their efficacy as antitoxoplasmic agents varied depending on the nature and position of the various substituents on the phenyl ring of their benzyl group. p-Cyano-6-benzylthioinosine and 2,4-dichloro-6-benzylthioinosine were the best ligands. In general, analogues with substitution at the para position of the phenyl ring were better ligands than those with the same substitutions at the meta or ortho position. The better binding of the para-substituted analogues is attributed to the combined effect of hydrophobic as well as van der Waals interactions. The 6-benzylthioinosine analogues were devoid of host-toxicity but all showed selective anti-toxoplasmic effect in cell culture and animal models. These results further confirm that toxoplasma adenosine kinase is an excellent target for chemotherapy and that 6-substituted purine nucleosides are potential selective antitoxoplasmic agents.

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.

Nanoparticles with specific bioadhesive properties to circumvent the pre-systemic degradation of fluorinated pyrimidines

The aim was to evaluate the potential of specific bioadhesive nanoparticles to increase the oral bioavailability of pre-systemic degraded drugs, using 5-fluorouridine (FURD) as model. For this purpose, poly(methylvinylether-co-maleic anhydride) nanoparticles (NP), NP coated with albumin (BSA-NP) and NP treated with albumin and 1,3-diaminopropane (BD-NP) were used. All the formulations displayed a similar size and drug loading. However, BSA-NP showed a tropism for the stomach, NP developed adhesive interactions with both the stomach and middle portions of the small intestine and BD-NP with the distal regions of the small intestine. These formulations were orally administered to laboratory animals and the FURD levels in plasma, tissues and urine were quantified at different times. From the urine data, the FURD bioavailability when loaded in either BSA-NP or NP was about 79% and 21%, respectively. For the control oral solution and BD-NP this parameter was 11% and 2%, respectively. FURD metabolism in gut was assessed by HPLC analysis of the lumen content. A FURD metabolite was found. Comparing the three nanoparticle formulations, the presence of the metabolite in the lumen contents was significantly higher for BD-NP than for NP and BSA-NP. In summary, the use of bioadhesive nanoparticles with tropism for the stomach mucosa may be considered as an adequate alternative to increase the bioavailability of some pre-systemic metabolised drugs.

Barbiturase, a novel zinc-containing amidohydrolase involved in oxidative pyrimidine metabolism

Barbiturase, which catalyzes the reversible amidohydrolysis of barbituric acid to ureidomalonic acid in the second step of oxidative pyrimidine degradation, was purified to homogeneity from Rhodococcus erythropolis JCM 3132. The characteristics and gene organization of barbiturase suggested that it is a novel zinc-containing amidohydrolase that should be grouped into a new family of the amidohydrolases superfamily. The amino acid sequence of barbiturase exhibited 48% identity with that of herbicide atrazine-decomposing cyanuric acid amidohydrolase but exhibited no significant homology to other proteins, indicating that cyanuric acid amidohydrolase may have evolved from barbiturase. A putative uracil phosphoribosyltransferase gene was found upstream of the barbiturase gene, suggesting mutual interaction between pyrimidine biosynthesis and oxidative degradation. Metal analysis with an inductively coupled radiofrequency plasma spectrophotometer revealed that barbiturase contains approximately 4.4 mol of zinc per mol of enzyme. The homotetrameric enzyme had K(m) and V(max) values of 1.0 mm and 2.5 micromol/min/mg of protein, respectively, for barbituric acid. The enzyme specifically acted on barbituric acid, and dihydro-l-orotate, alloxan, and cyanuric acid competitively inhibited its activity. The full-length gene encoding the barbiturase (bar) was cloned and overexpressed in Escherichia coli. The kinetic parameters and physicochemical properties of the cloned enzyme were apparently similar to those of the wild-type.

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.