Towards species-specific antifolates.

Dihydrofolate reductase (DHFR) plays an essential role in cellular biochemistry and has been a well-recognized drug target for over fifty years. Antifolate inhibitors of DHFR, including clinically used therapeutics such as methotrexate, trimethoprim, and pyrimethamine have been successful as anticancer, antibacterial, antifungal and antiparasitic agents. As resistant strains of these microorganisms evolve and as new disease threats arise, the need for new antifolates that are potent and specific for infectious organisms becomes more pressing. Several new antifolates have been reported over the past decade; many of these are potent against a particular species of DHFR, but achieving the goal of potency and selectivity has proven to be more difficult. This review will describe recent advances in attaining species selectivity in developing new antifolates. Specifically, advances in developing inhibitors against Pneumocystis jirovecii and Plasmodium falciparum, the causative agents in pneumocystis pneumonia and malaria, respectively, will be presented.

Phase I trial of trimetrexate glucuronate on a five-day bolus schedule: clinical pharmacology and pharmacodynamics.

Trimetrexate glucuronate (TMTX), a nonclassical folate antagonist, has been evaluated clinically on several schedules. We have studied TMTX administered as an iv bolus for 5 consecutive days every 3 weeks in 35 patients with advanced solid tumors. Drug was given at doses ranging from 7.6 to 18.8 mg/m2. The maximal tolerated dose was 13.1 mg/m2 per day x 5 for patients without prior myelotoxic treatment and 7.6 mg/m2 per day x 5 for previously treated patients. Because of wide individual differences in drug tolerance, dose escalation in 25% increments is recommended for patients not experiencing toxic effects. The dose-limiting toxicity was neutropenia. Rash and mucositis were also significant. TMTX concentrations were measured 1 and 24 hours after each dose, and the data were fit by use of a one-compartment pharmacokinetic model. With this simplified sampling and modeling scheme, the mean total body clearance (+/- SD) of trimetrexate was 31 +/- 20 mL/min per m2 and the volume of distribution was 13 +/- 7 L/m2. Mean plasma concentrations 1 hour after a dose were 1.12, 2.43, 3.33, and 3.25 mumol/L at 7.6, 9.1, 10.9, and 13.1 mg/m2, respectively. The mean TMTX concentration (+/- SD) 24 hours after a dose was 114 +/- 87 nmol/L. The mean area under the concentration-versus-time curve at 13.1 mg/m2 was 2,266 mumol.min/L. The drug concentration 1 hour after the first dose and the area under the concentration-versus-time curve were highly correlated with leukopenia and thrombocytopenia (r = .6 and .65 and P = .0007 and .0001, respectively). The maximal tolerated dose on the daily x 5 schedule was 30% of the dose tolerated on an iv bolus schedule. The choice of drug schedule for clinical trials when murine and human pharmacokinetics differ is discussed. Phase II trials are under way with both the iv bolus and the daily x 5 schedules.

New small-molecule inhibitors of dihydrofolate reductase inhibit Streptococcus mutans.

Streptococcus mutans is a major aetiological agent of dental caries. Formation of biofilms is a key virulence factor of S. mutans. Drugs that inhibit S. mutans biofilms may have therapeutic potential. Dihydrofolate reductase (DHFR) plays a critical role in regulating the metabolism of folate. DHFR inhibitors are thus potent drugs and have been explored as anticancer and antimicrobial agents. In this study, a library of analogues based on a DHFR inhibitor, trimetrexate (TMQ), an FDA-approved drug, was screened and three new analogues that selectively inhibited S. mutans were identified. The most potent inhibitor had a 50% inhibitory concentration (IC50) of 454.0±10.2nM for the biofilm and 8.7±1.9nM for DHFR of S. mutans. In contrast, the IC50 of this compound for human DHFR was ca. 1000nM, a >100-fold decrease in its potency, demonstrating the high selectivity of the analogue. An analogue that exhibited the least potency for the S. mutans biofilm also had the lowest activity towards inhibiting S. mutans DHFR, further indicating that inhibition of biofilms is related to reduced DHFR activity. These data, along with docking of the most potent analogue to the modelled DHFR structure, suggested that the TMQ analogues indeed selectively inhibited S. mutans through targeting DHFR. These potent and selective small molecules are thus promising lead compounds to develop new effective therapeutics to prevent and treat dental caries.

2,4-Diamino-5-chloroquinazoline analogues of trimetrexate and piritrexim: synthesis and antifolate activity.

Ten heretofore undescribed 2,4-diamino-5-chloroquinazoline analogues of trimetrexate (TMQ) and piritrexim (PTX) were synthesized and tested as inhibitors of dihydrofolate reductase (DHFR) from rat liver, Pneumocystis carinii, and Toxoplasma gondii. The most active quinazolines against both the P. carinii and the T. gondii enzyme were those with an ArCH2-NH or ArNHCH2 side chain. Among ArNH(CH2)n compounds with n = 1-3 and either 2',5'-dimethoxyphenyl or 3',4',5'-trimethoxyphenyl as the Ar moiety, activity decreased in the order n = 1 > n = 2 > n = 3. The best inhibitor of P. carinii DHFR, 2,4-diamino-5-chloro-6-[(N-methyl-3',4',5'-trimethoxyanilino)methy l] quinazoline (10) had an IC50 of 0.012 microM and was slightly more potent than TMQ and PTX. Compound 10 was also the best inhibitor of T. gondii DHFR, with an IC50 of 0.0064 microM corresponding again to a minor increase in activity over TMQ and PTX. However, as with these standard agents, 10 showed no appreciable selectivity for either the P. carinii or T. gondii enzyme relative to the rat liver enzyme. The highest selectivity achieved in this limited series was with 2,4-diamino-5-chloro-6-[N-(3',4',5'-trimethoxybenzyl)-N-methylamino] quinazoline (17) against T. gondii DHFR. While 17 (IC50 = 0.016 microM) was somewhat less potent than 10, its selectivity, as defined by the ratio IC50(rat liver)/IC50(T. gondii) was ca. 30-fold higher than that of TMQ or PTX. Two compounds, 2,4-diamino-5-chloro-6-[(3',4',5'-trimethoxyanilino)methyl] quinazoline (9) and 2,4-diamino-5-chloro-6-[N-(3',4',5'-trimethoxybenzyl) amino]quinazoline (15), were also tested against human DHFR and were found to have an IC50/[E] of 0.5, indicating that their binding was near-stoichiometric.

Synthesis and biological activities of conformationally restricted, tricyclic nonclassical antifolates as inhibitors of dihydrofolate reductases.

Seven novel tricyclic pyrimido[4,5-c][2,7]naphthyridones 5-8 and the corresponding naphthyridines 9-11 were synthesized as conformationally restricted inhibitors of dihydrofolate reductase (DHFR) and as antitumor and/or antiinfectious agents. The analogues were designed to orient the side chain trimethoxyphenyl group in different conformationally defined positions in order to explore the effect of the side chain orientation on binding affinity and selectivity for DHFR from various species. The semirigid orientations were achieved by bridging the C5 and N10 of compound 12 with a N-ethyl bridge and by variation of the position of double bonds in rings B and C as well as substitution at the 2',6'-positions of the phenyl ring. The synthesis of compounds 5-11 were accomplished by cyclocondensation of the appropriate keto ester (as the biselectrophile) with 2,4,6-triaminopyrimidine to afford the lactam 5. The dehydrolactams 6 and 7 were prepared by air oxidation and PtO2-catalyzed dehydrogenation of 7, respectively. The dichloro dehydro lactam 8 was obtained by refluxing lactam 5 and/or 6 in POCl3 or a mixture of POCl3/PCl5. Compounds 9-11 were obtained by two methods, direct borane reduction of lactam 5 or 6 or thiation of the dipivoylated lactam 15 followed by reductive dethiation. Compounds 9-11 were interconverted by air oxidation or PtO2-catalyzed reduction/oxidation, respectively. The compounds were evaluated as inhibitors of DHFR from Pneumocystis carinii (pc) and Toxoplasma gondii (tg) with rat liver (rl) serving as the reference mammalian enzyme. In the lactam series 5-8, the most unsaturated analogue 7 showed an IC50 of 86 nM against rlDHFR, almost 100-fold more active than 5 and 3-fold more active than 6. The 2',6'-dichloro dehydro lactam 8 was less active than the corresponding dehydro lactam 6 against rlDHFR. In the naphthyridine series 9-11, the dehydro analogue 10 was more active than 9 against rlDHFR. The fully reduced analogue 11 (as a mixture of cis and trans isomers) was the most active in the naphthyridine series. The analogues were, in general, more inhibitory against rlDHFR than against pcDHFR, or tgDHFR, and thus lacked selectivity. In addition, they were less potent than the bicyclic compounds trimetrexate 3 (TMQ) and piritrixim 4 (PTX).

2,4-Diaminothieno[2,3-d]pyrimidine analogues of trimetrexate and piritrexim as potential inhibitors of Pneumocystis carinii and Toxoplasma gondii dihydrofolate reductase.

A series of eight previously undescribed 2,4-diaminothieno[2,3-d]pyrimidine analogues of the potent dihydrofolate reductase (DHFR) inhibitors trimetrexate (TMQ) and piritrexim (PTX) were synthesized as potential drugs against Pneumocystis carinii and Toxoplasma gondii, which are major causes of severe opportunistic infections in AIDS patients. 2,4-Diamino-5-methyl-6-(aryl/aralkyl)thieno[2,3-d]pyrimidines with 3,4,5-trimethoxy or 2,5-dimethoxy substitution in the aryl/aralkyl moiety and 2,4-diamino-5-(aryl/aralkyl)thieno[2,3-d]pyrimidines with 2,5-dimethoxy substitution in the aryl/aralkyl moiety were obtained by reaction of the corresponding 2-amino-3-cyanothiophenes with chloroformamidine hydrochloride. The aryl group in the 5,6-disubstituted analogues was either attached directly to the hetero ring or was separated from it by one or two carbons, whereas the aryl group in the 5-monosubstituted analogues was separated from the hetero ring by two or three carbons. 2-Amino-3-cyano-5-methyl-6-(aryl/alkyl)thiophene intermediates for the preparation of the 5,6-disubstituted analogues were prepared from omega-aryl-2-alkylidene-malononitriles and sulfur in the presence of a secondary amine, and 2-amino-3-cyano-4-(aryl/aralkyl)thiophene intermediates for the preparation of the 5-monosubstituted analogues were obtained from omega-aryl-1-chloro-2-alkylidenemalononitriles and sodium hydrosulfide. Synthetic routes to the heretofore unknown ylidenemalononitriles, and the ketone precursors thereof, were developed. The final products were tested in vitro as inhibitors of DHFR from Pneumocystis carinii, Toxoplasma gondii, rat liver, beef liver, and Lactobacillus casei. A selected number of previously known 2,4-diaminothieno[2,3-d]pyrimidines lacking the 3,4,5-trimethoxyphenyl and 2,5-dimethoxyphenyl substitution pattern of TMQ and PTX, respectively, were also tested for comparison. None of the compounds was as potent as TMQ or PTX, and while some of them showed some selectivity in their binding to Pneumocystis carinii and Toxoplasma gondii versus rat liver DHFR, this effect was not deemed large enough to warrant further preclinical evaluation.

A phase I trial of trimetrexate glucuronate (NSC 352122) given every 3 weeks: clinical pharmacology and pharmacodynamics.

Trimetrexate glucuronate (TMTX), a non-classic folate antagonist, has been evaluated clinically on several schedules. We studied TMTX given as an i.v. bolus over 5-30 min every 3 weeks in 44 patients with advanced solid tumors; it was given at doses ranging from 20 to 275 mg/m2. The maximal tolerated dose (MTD) on this schedule is 220 mg/m2, which we also recommend as a starting dose for phase II studies in patients without extensive prior therapy. Because of wide individual differences in drug tolerance, dose escalation in 25% increments is recommended for non-toxic patients. The principal dose-limiting toxicity was myelosuppression, although in some patients a flu-like syndrome precluded dose escalation. Significant rash and mucositis also frequently occurred in toxic patients. TMTX plasma concentrations were measured after the first dose and the data were fit by two- or three-compartment mammillary pharmacokinetic models. The TMTX clearance rate was 36.5 +/- 21 ml/min per m2 and did not change with dose; non-linearities with increasing dose were apparent in the steady-state volume of distribution (Vss) and in the terminal disposition half-life (t1/2). The difference between pre-treatment and nadir leucocyte counts was correlated with TMTX dose (r = 0.58; P = 0.0006) and with the area under the concentration vs time curve (AUC) (r = 0.41; P = 0.02). Pre-treatment plasma albumin concentrations correlated weakly with the nadir white blood count (r = -0.36; P = 0.047). Optimal schedules for the administration of TMTX have not been established and phase II trials using both bolus and daily X 5 schedules are under way.

Trimetrexate glucuronate associated with anti-Kaposi sarcoma effect.

Trimetrexate glucuronate, a dihydrofolate reductase inhibitor related to methotrexate, was developed by Parke-Davis as an alternative antineoplastic agent for tumors, especially sarcomas, that had developed resistance to methotrexate. This is a report on a patient with AIDS who developed Pneumocystis carinii pneumonia, which was treated with trimethoprim sulfamethoxazole (Bactrim) with poor response, then with pentamidine with poor response, and finally with trimetrexate glucuronate (Neutrexin) and leucovorin rescue, with good response. The patient also suffered from cutaneous and visceral Kaposi's sarcoma (KS), which had been treated with high- dose HCG1 and well recognized chemotherapeutic protocols. Both HCG and chemotherapy resulted in tumor regression. The patient's KS flared, however, when he developed pneumocystis pneumonia. When trimetrexate glucuronate and leucovorin rescue were administered, his tumor burden decreased significantly, suggesting that trimetrexate glucuronate may have some activity against KS. The regression of KS in this anecdotal observation may be secondary to a delayed response from HCG and/or chemotherapy, or secondary to a spontaneous partial regression. Such regression may only be of the decreased edema around the KS lesions and not the neoplastic tissue itself. If other clinicians see this same phenomenon, however, it is possible that trimetrexate glucuronate may have an anti-KS effect. Such future clinical observations would warrant further testing at the basic science level.

Lipophilic antifolate trimetrexate is a potent inhibitor of Trypanosoma cruzi: prospect for chemotherapy of Chagas' disease.

Trypanosoma cruzi, a protozoan parasite, is the causative agent for Chagas' disease, which poses serious public health problem in Latin America. The two drugs available for the treatment of this disease are effective only against recent infections and are toxic. Dihydrofolate reductase (DHFR) has a proven track record as a drug target. The lipophilic antifolate trimetrexate (TMQ), which is an FDA-approved drug for the treatment of Pneumocystis carinii infection in AIDS patients, is a potent inhibitor of T. cruzi DHFR activity, with an inhibitory constant of 6.6 nM. The compound is also highly effective in killing T. cruzi parasites. The 50 and 90% lethal dose values against the trypomastigote are 19 and 36 nM, and the corresponding values for the amastigote form are 26 and 72 nM, respectively. However, as TMQ is also a good inhibitor of human DHFR, further improvement of the selectivity of this drug would be preferable. Identification of a novel antifolate selective against T. cruzi would open up new therapeutic avenues for treatment of Chagas' disease.

Analysis of two polymorphic forms of a pyrido[2,3-d]pyrimidine N9-C10 reversed-bridge antifolate binary complex with human dihydrofolate reductase.

The results of the crystal structure determination of human dihydrofolate reductase (hDHFR) as a binary complex with the potent N9-C10 reversed-bridge antifolate inhibitor 2,4-diamino-6-[N-(3',4',5'-trimethoxybenzyl)-N-methylamino]pyrido[2,3-d]pyrimidine (1) are reported for two independent polymorphic rhombohedral R3 lattices [R3(1) and R3(2)]. Data from these two crystal forms were refined to 1.90 A resolution for complex R3(1), with R = 0.186 for 9689 data, and to 1.80 A resolution for complex R3(2), with R = 0.194 for 13 305 data. Changes in the loop geometry between the two structures reflects contact differences in the packing environments in the two R3 lattices. The largest changes (between 0.5 and 1.7 A) are observed for the loop regions encompassing residues 16-25, 40-48, 81-89, 99-108, 143-148 and 161-169. Comparison of the intermolecular contacts of these loops reveals that the R3(2) lattice is more tightly packed, as reflected in its smaller V(M) value and smaller solvent content. The conformation of inhibitor (1) is similar in both structures and the N9-C10 bridge geometry is more similar to that observed for the normal C9-N10 bridge of trimetrexate (TMQ) than to the other N9-C10 reversed-bridge antifolates previously reported. The effect of the N9-C10 reversed-bridge geometry is to distort the bridge from coplanarity with the pyrido[2,3-d]pyrimidine ring system and to twist the C10 methylene conformation towards a gauche conformation. This also influences the conformation of the methoxybenzyl ring, moving it away from a trans position and placing the 5'-methoxy group deeper within the hydrophobic pocket made by Leu60, Pro61 and Asn64 of the hDHFR active site.

Identification of highly potent and selective inhibitors of Toxoplasma gondii dihydrofolate reductase.

Toxoplasma gondii RH was obtained in high yield from culture in RPMI medium on a line of Chinese hamster ovary cells lacking dihydrofolate reductase activity (ATCC 3952 dhfr-; American Type Culture Collection). Dihydrofolate reductase preparations from harvested organisms had specific activities of 22.9 +/- 2.1 nmol/min/mg. The 50% inhibitory concentrations against reference compounds were 0.014 microM for methotrexate, 0.24 microM for pyrimethamine, 2.7 microM for trimethoprim, and 0.010 microM for trimetrexate. The Km value for NADPH was 11 microM and followed Michaelis-Menten kinetics; the Km for dihydrofolate was ca. 11 microM, but substrate inhibition appeared to occur at high substrate concentrations. Dihydrofolate reductase from T. gondii was used to screen 130 compounds from the National Cancer Institute repository. Thirteen compounds were > 100-fold more potent than pyrimethamine toward T. gondii dihydrofolate reductase; six compounds with various potencies were 8 to 46 times as selective as pyrimethamine for the protozoal form of the enzyme over the mammalian form. Four trimetrexate analogs were more potent than trimetrexate, and two were significantly more selective. Representative compounds were also tested in a culture model of T. gondii employing uracil incorporation as an index of growth. One pyrimethamine analog was as effective as pyrimethamine in inhibiting T. gondii in culture (50% inhibitory concentration, 0.45 microM). Three other compounds were also effective at micromolar concentrations.

Separation and determination of the antitumor drug piritrexim by molecularly imprinted microspheres in high-performance liquid chromatography.

Molecularly imprinted microspheres were synthesised using the antitumor drug piritrexim (PTX) as a template molecule by aqueous microsuspension polymerisation and were used as a high-performance liquid chromatographic stationary phase. The molecularly imprinted column exhibited strong retention behaviour to the template molecule. The influences of pH of the buffer and the ratio of methanol to buffer on the retention behaviour were investigated in detail. Results indicated that the baseline separation of PTX, trimetrexate (TMX), trimethoprim (TMP) and sulfamethazine (SMZ) was achieved when the pH value of the acetate buffer was above pH 3.5 and the ratio of methanol to the buffer was 6:4 (v/v). A gradient elution programme was employed to enhance the separation, which led to an improvement in sensitivity and a reduction in determination errors. The method developed was used to analyse urine samples supplemented with PTX. The recoveries of 5 microg mL(-1) PTX in the urine sample were 99.1+/-3.0% and 93.3+/-2.8% at the beginning and 24 h later, respectively.

Low-dose trimetrexate glucuronate and protracted 5-fluorouracil infusion in previously untreated patients with advanced pancreatic cancer.

BACKGROUND: 5-Fluorouracil (5-FU)-based regimens have not been shown to prolong survival or provide clinical benefit in patients with advanced pancreatic cancer. The purpose of this study was to determine the tolerability of protracted venous infusion (PVI) of 5-FU, modulated by a low dose of the synthetic antifolate trimetrexate, in patients with advanced pancreatic cancer. PATIENTS AND METHODS: Twenty-three chemotherapy-naïve patients were evaluated. Patients were enrolled in four consecutive cohorts in which the weekly dose of trimetrexate was escalated in 10 mg/m2 increments, from 20 to 50 mg/m2. PVI 5-FU was administered at a fixed dose of 225 mg/m2/day. Treatment was administered for 6 successive weeks, every 8 weeks. RESULTS: Twenty-two patients were assessable. The maximum tolerated dose of trimetrexate was 40 mg/m2. The most common grade 3 and 4 toxicity was diarrhea. There were no treatment-related deaths. Preliminary analysis of activity revealed a response rate of 9%, with 41% of the patients having stable disease for a median duration of 3.8 months. The median survival for the entire group was 6.9 months (range 1-29 months). A clinical benefit response was experienced by 27.2% of patients. CONCLUSIONS: Low-dose trimetrexate can be safely administered in combination with PVI 5-FU. This treatment is well tolerated and is associated with palliative activity in advanced pancreatic cancer.

Inhibition of Pneumocystis dihydrofolate reductase by analogs of pyrimethamine, methotrexate and trimetrexate.

Dihydrofolate reductase was obtained from Pneumocystis carinii isolated from heavily infected lungs of female Sprague-Dawley rats infected by transtracheal inoculation. The enzyme differed significantly from other forms of dihydrofolate reductase in response to KCl and to antifolate drugs. Dihydrofolate reductase from P. carinii was used to assess activity of analogs of pyrimethamine, methotrexate, and trimetrexate. One pyrimethamine analog was selective for P. carinii dihydrofolate reductase; potency was in the micromolar range. In contrast, 21 methotrexate analogs and 2 trimetrexate analogs were selective for P. carinii dihydrofolate reductase; potencies for these were in the nanomolar range.

Transient in vivo selection of transduced peripheral blood cells using antifolate drug selection in rhesus macaques that received transplants with hematopoietic stem cells expressing dihydrofolate reductase vectors.

One of the main obstacles for effective human gene therapy for hematopoietic disorders remains the achievement of an adequate number of genetically corrected blood cells. One approach to this goal is to incorporate drug resistance genes into vectors to enable in vivo selection of hematopoietic stem cells (HSCs). Although a number of drug resistance vectors enable HSC selection in murine systems, little is known about these systems in large animal models. To address this issue, we transplanted cells transduced with dihydrofolate resistance vectors into 6 rhesus macaques and studied whether selection of vector-expressing cells occurred following drug treatment with trimetrexate and nitrobenzylmercaptopurineriboside-phosphate. In some of the 10 administered drug treatment courses, substantial increases in the levels of transduced peripheral blood cells were noted; however, numbers returned to baseline levels within 17 days. Attempts to induce stem cell cycling with stem cell factor and granulocyte-colony stimulating factor prior to drug treatment did not lead to sustained enrichment for transduced cells. These data highlight an important species-specific difference between murine and nonhuman primate models for assessing in vivo HSC selection strategies and emphasize the importance of using drugs capable of inducing selective pressure at the level of HSCs.