Modulation of plasma uridine concentration by 5-(phenylselenenyl)acyclouridine, an inhibitor of uridine phosphorylase: relevance to chemotherapy

DHODH inhibition impedes glioma stem cell proliferation, induces DNA damage, and prolongs survival in orthotopic glioblastoma xenografts

Glioma stem cells (GSCs) promote tumor progression and therapeutic resistance and exhibit remarkable bioenergetic and metabolic plasticity, a phenomenon that has been linked to their ability to escape standard and targeted therapies. However, specific mechanisms that promote therapeutic resistance have been somewhat elusive. We hypothesized that because GSCs proliferate continuously, they may require the salvage and de novo nucleotide synthesis pathways to satisfy their bioenergetic needs. Here, we demonstrate that GSCs lacking EGFR (or EGFRvIII) amplification are exquisitely sensitive to de novo pyrimidine synthesis perturbations, while GSCs that amplify EGFR are utterly resistant. Furthermore, we show that EGFRvIII promotes BAY2402234 resistance in otherwise BAY2402234 responsive GSCs. Remarkably, a novel, orally bioavailable, blood-brain-barrier penetrating, dihydroorotate dehydrogenase (DHODH) inhibitor BAY2402234 was found to abrogate GSC proliferation, block cell-cycle progression, and induce DNA damage and apoptosis. When dosed daily by oral gavage, BAY2402234 significantly impaired the growth of two different intracranial human glioblastoma xenograft models in mice. Given this observed efficacy and the previously established safety profiles in preclinical animal models and human clinical trials, the clinical testing of BAY2402234 in patients with primary glioblastoma that lacks EGFR amplification is warranted.

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.

Functional analysis of pyrimidine biosynthesis enzymes using the anticancer drug 5-fluorouracil in Caenorhabditis elegans

Pyrimidine biosynthesis enzymes function in many cellular processes and are closely associated with pyrimidine antagonists used in cancer chemotherapy. These enzymes are well characterized from bacteria to mammals, but not in a simple metazoan. To study the pyrimidine biosynthesis pathway in Caenorhabditis elegans, we screened for mutants exhibiting resistance to the anticancer drug 5-fluorouracil (5-FU). In several strains, mutations were identified in ZK783.2, the worm homolog of human uridine phosphorylase (UP). UP is a member of the pyrimidine biosynthesis family of enzymes and is a key regulator of uridine homeostasis. C. elegans UP homologous protein (UPP-1) exhibited both uridine and thymidine phosphorylase activity in vitro. Knockdown of other pyrimidine biosynthesis enzyme homologs, such as uridine monophosphate kinase and uridine monophosphate synthetase, also resulted in 5-FU resistance. Uridine monophosphate kinase and uridine monophosphate synthetase proteins are redundant, and show different, tissue-specific expression patterns in C. elegans. Whereas pyrimidine biosynthesis pathways are highly conserved between worms and humans, no human thymidine phosphorylase homolog has been identified in C. elegans. UPP-1 functions as a key regulator of the pyrimidine salvage pathway in C. elegans, as mutation of upp-1 results in strong 5-FU resistance.

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.

Coordination of thiouridine monophosphates with selected metal ions

Potentiometric and spectroscopic data obtained for the complexes of two thio-substituted uridine-monophosphates with Cu(2+), Ni(2+) and Cd(2+) ions have shown that both thionucleotide are more effective ligands than their nucleoside analogues. The basic binding site for all metal ions is the sulfur atom. The chelation by adjacent N(3) donor is also likely, although unfavorable four-membered chelate ring is formed.

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.

Effect of administration of 5-(phenylselenenyl)acyclouridine, an inhibitor of uridine phosphorylase, on the anti-tumor efficacy of 5-fluoro-2'-deoxyuridine against murine colon tumor C26-10

The effect of co-administration of 5-(phenylselenenyl)acyclouridine (PSAU), a new uridine phosphorylase (UrdPase, EC 2.4.2.3) inhibitor, on the efficacy of 5-fluoro-2'-deoxyuridine (FdUrd) was tested against murine colon C26-10 tumor xenografts. In contrast to our previous results with human tumors, co-administration of PSAU with FdUrd decreased instead of increasing the efficacy of FdUrd against tumor growth. However, co-administration of PSAU with FdUrd (300 mg/kg/day) protected the mice completely from the 83% mortality induced by the same dose of FdUrd alone. Enzyme studies indicated that UrdPase in colon C26-10 tumors is responsible for the catabolism of FdUrd to 5-fluorouracil (FUra), as colon C26-10 tumors do not have thymidine phosphorylase (dThdPase, EC 2.4.2.4). In contrast, colon C26-10 tumors had extraordinarily high UrdPase activity (300 micromol/min/mg protein), which was at least 200-fold higher than the highest UrdPase activity in any of the human xenografts we tested previously. Furthermore, the activities of UrdPase and orotate phosphoribosyltransferase (OPRTase, EC 2.4.2.10) were 192- and 2-fold higher, respectively, while that of dihydrouracil dehydrogenase (EC 1.3.1.2) was 1000-fold lower in the tumor than in the host liver. It is suggested that FdUrd exerts its anticancer effects against colon C26-10 tumors mainly through the catabolism of FdUrd to FUra by UrdPase, which then could be anabolized to 5-fluorouridine 5'-monophosphate (FUMP) by OPRTase and ultimately to other toxic 5-fluorouridine nucleotides, hence inducing the observed FdUrd toxic effects. Co-administration of PSAU with FdUrd inhibited UrdPase and the catabolism of FdUrd to FUra. This would result in the observed reduction of the antitumor efficacy of FdUrd. In addition, the increase in plasma uridine concentration induced by PSAU as well as the catabolism of FUra by the high dihydrouracil dehydrogenase activity in the liver also may have circumvented any residual FUra toxic effects against the host. These results clearly demonstrate that the anticancer efficacy of the combination of UrdPase inhibitors and FdUrd is not general and is dependent largely on the type of tumor under treatment and the mode of FdUrd metabolism in these tumors.

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.