Inhibition of polyamine biosynthesis by alpha-difluoromethylornithine in African trypanosomes and Pneumocystis carinii as a basis of chemotherapy: biochemical and clinical aspects

An international randomised trial of celecoxib versus celecoxib plus difluoromethylornithine in patients with familial adenomatous polyposis

BACKGROUND AND AIM

Although Non-steroidal anti-inflammatory drugs reduce colorectal adenoma burden in familial adenomatous polyposis (FAP), the utility of combining chemopreventive agents in FAP is not known. We conducted a randomised trial of celecoxib (CXB) versus CXB+diflouromethylornithine (DFMO) to determine the synergistic effect, if any.

METHODS

The primary endpoint was % change in adenoma count in a defined field. Secondary endpoints were adenoma burden (weighted by adenoma diameter) and video review of entire colon/rectal segments. Adverse event (AEs) were monitored by National Cancer Institution toxicity criteria.

RESULTS

112 subjects were randomised: 60 men and 52 women at a mean age of 38 years. For the 89 patients who had landmark-matched polyp counts available at baseline and 6 months, the mean % change in adenoma count over the 6 months of trial was -13.0% for CXB+DFMO and -1.0% for CXB (p=0.69). Mean % change in adenoma burden was -40% (CXB+DFMO) vs -27% (CXB) (p=0.13). Video-based global polyp change was -0.80 for CXB+DFMO vs -0.33 for CXB (p=0.03). Fatigue was the only significant AE, worse on the CXB arm (p=0.02).

CONCLUSIONS

CXB combined with DFMO yielded moderate synergy according to a video-based global assessment. No significant difference in adenoma count, the primary endpoint, was seen between the two study arms. No evidence of DFMO-related ototoxicity was seen. There were no adverse cardiovascular outcomes in either trial arm and no significant increase in AEs in the CXB+DFMO arm of the trial. Differences in outcomes between primary and secondary endpoints may relate to sensitivity of the endpoint measures themselves.

TRIAL REGISTRATION NUMBER

ClinicalTrials.gov number N01-CN95040.

Use of Fluorinated Functionality in Enzyme Inhibitor Development: Mechanistic and Analytical Advantages

On the one hand, owing to its electronegativity, relatively small size, and notable leaving group ability from anionic intermediates, fluorine offers unique opportunities for mechanism-based enzyme inhibitor design. On the other, the "bio-orthogonal" and NMR-active 19-fluorine nucleus allows the bioorganic chemist to follow the mechanistic fate of fluorinated substrate analogues or inhibitors as they are enzymatically processed. This article takes an overview of the field, highlighting key developments along these lines. It begins by highlighting new screening methodologies for drug discovery that involve appropriate tagging of either substrate or the target protein itself with (19)F-markers, that then report back on turnover and binding, respectively, via an the NMR screen. Taking this one step further, substrate-tagging with fluorine can be done is such a manner as to provide stereochemical information on enzyme mechanism. For example, substitution of one of the terminal hydrogens in phosphoenolpyruvate, provides insight into the, otherwise latent, facial selectivity of C-C bond formation in KDO synthase. Perhaps, most importantly, from the point of view of this discussion, appropriately tailored fluorinated functionality can be used to form to stabilized "transition state analogue" complexes with a target enzymes. Thus, 5-fluorinated pyrimidines, alpha-fluorinated ketones, and 2-fluoro-2-deoxysugars each lead to covalent adduction of catalytic active site residues in thymidylate synthase, serine protease and glycosidase enzymes, respectively. In all such cases, (19)F NMR allows the bioorganic chemist to spectrally follow "transition state analogue" formation. Finally, the use of specific fluorinated functionality to engineer "suicide substrates" is highlighted in a discussion of the development of the alpha-(2'Z-fluoro)vinyl trigger for amino acid decarboxylase inactivation. Here (19)F NMR allows the bioorganic chemist to glean useful partition ratio data directly out of the NMR tube.

A plethora of targets, a paucity of drugs: progress towards the development of novel chemotherapies for human African trypanosomiasis

Human African trypanosomiasis is a major health problem in large regions of Africa. Current chemotherapeutic options are limited and far from ideal. A diverse range of drug targets has been identified and validated in trypanosomes. These include several organelles (glycosomes, acidocalcisomes, kinetoplast) that are not represented in the mammalian host and biochemical pathways that differ significantly from host counterparts (carbohydrate metabolism, protein and lipid modification, response to oxidative stress, cell cycle). However, there has been little progress in developing novel drugs. Pharmaceutical companies are unwilling to invest in the development of drugs for a market that comprises some of the worlds poorest people. This review highlights some of the most attractive drug targets in trypanosomes.

The efficacy of ascofuranone in a consecutive treatment on Trypanosoma brucei brucei in mice

Consecutive administration of ascofuranone without glycerol was found to have therapeutic efficacy against Trypanosoma brucei brucei infection in mice. A suspension of ascofuranone (25-100 mg/kg) was administrated intraperitoneally every 24 h for 1-4 consecutive days to trypanosome-infected mice and efficacy was compared with oral treatment. With intraperitoneal administration, all mice treated with 100 mg/kg ascofuranone for 4 consecutive days were cured. On contrary, with oral treatment a higher dose of ascofuranone (400 mg/kg) was needed for 8 consecutive days to cure the mice. With intraperitoneal treatment, parasitemia was strongly suppressed, with almost all long slender bloodstream forms of the parasite changed to short stumpy forms by day 3 and the parasites have been eliminated 4 days after the start of treatment. These ascofuranone-induced short stumpy forms were morphologically analogous to the stumpy forms 2 days after peak parasitemia of pleomorphic clone of T. b. brucei GUTat 3.1. However, the properties of ubiquinol oxidase activity, which is the target of ascofuranone, in mitochondria isolated from before and after treatment, were almost same. The enzymatic activities of ubiquinol oxidase were only decreased to approximately 30% within a day after treatment, and then kept at nearly the same level. In the present study, we have improved regimen for administration of ascofuranone without glycerol, and demonstrated that consecutively administrated ascofuranone showed trypanocidal effects in T. b. brucei infected mice. Our present results strongly suggest that consecutive administration of ascofuranone may be an effective chemotherapy for African trypanosomiasis.

Furazolidone and nitrofurantoin in the treatment of experimental Pneumocystis carinii pneumonia

Furazolidone and nitrofurantoin, oral nitrofuran derivatives with broad-spectrum antimicrobial properties already in clinical use, were compared for activity against Pneumocystis carinii in an immunosuppressed rat model of P. carinii pneumonia. Furazolidone exhibited only slight activity as a prophylactic agent but was moderately effective in the therapy of pneumocystosis. The median histologic score and organism count fell from 4+ and 10(8) to 10(9) cysts per lung, respectively, in the controls to 1+ to 2+ and 10(7) to 10(8) cysts per lung, respectively, in the furazolidone-treated groups. However, these results were not as good as those obtained with the standard drug, trimethoprim-sulfamethoxazole (0+, 10(6) to 10(7) cysts per lung). Nitrofurantoin showed little anti-P. carinii activity despite different doses or drug preparations. The high doses of furazolidone used here and their toxic effects on the rats will probably discourage investigation of this drug in the treatment of pneumocystosis in humans. Nevertheless, since many nitrofurans have been synthesized, further exploration of this class of compounds might be helpful in developing new anti-P. carinii agents or in studying structure-activity relationships.

The selective toxicity of medications used in the treatment of AIDS on the CEM human leukemic CD4+ T-cell line

Eleven medications used in the treatment of the human immunodeficiency virus or subsequent opportunistic infections were tested to determine their toxicity to the growth of the CEM line of transformed CD4+ T-cells. A selectivity ratio (IC50/EC50) for each agent against its target pathogen was calculated as an in vitro indication of the therapeutic value of that agent. Among the anti-HIV agents tested in this study, 2',3'-dideoxyinosine (ddI) had the highest in vitro selectivity ratio, 2.5 times higher than that of 3'-azido-3'-deoxythymidine (AZT). Dapsone had the highest selectivity ratio of the four agents used in the treatment of Pneumocystis carinii pneumonia that were tested. Ketoconazole had a very low selectivity ratio for Candida spp. due to its very high toxicity to CD4+T-cells.

Cochlear damage and increased threshold in alpha-difluoromethylornithine (DFMO) treated guinea pigs

Alpha-difluoromethylornithine (DFMO) inhibits polyamine synthesis and is used as an antineoplastic and antiparasitic drug. In early human trials DFMO unexpectedly caused a sensorineural hearing loss. In the current study DFMO was administered to guinea pigs to investigate its effects on organ of Corti histology and on auditory thresholds. Histologic examination revealed that DFMO caused greatest damage in the hook and first turn. Damage in the second and third turns was minimal. Animals treated for 12 weeks with DFMO differed significantly (P less than 0.05) from controls in the hook and first turn in that: 1) DFMO caused a loss of hair cells in all rows. Loss of inner hair cells was greater than that of outer hair cells. 2) The remaining outer hair cells were shorter and contained a greater number of Hensen bodies. 3) The Deiters' cell bodies were longer and this increased length was associated with the decreased length of the corresponding outer hair cells. Brainstem audiometry showed that DFMO produced a hearing loss and the magnitude of this loss increased over twelve weeks of DFMO administration.

Metabolism and action of amino acid analog anti-cancer agents

The preclinical pharmacology, antitumor activity and toxicity of seven of the more important amino acid analogs, with antineoplastic activity, is discussed in this review. Three of these compounds are antagonists of L-glutamine: acivicin, DON and azaserine; and two are analogs of L-aspartic acid: PALA and L-alanosine. All five of these antimetabolites interrupt cellular nucleotide synthesis and thereby halt the formation of DNA and/or RNA in the tumor cell. The remaining two compounds, buthionine sulfoximine and difluoromethylornithine, are inhibitors of glutathione and polyamine synthesis, respectively, with limited intrinsic antitumor activity; however, because of their powerful biochemical actions and their low systemic toxicities, they are being evaluated as chemotherapeutic adjuncts to or modulators of other more toxic antineoplastic agents.

Eflornithine. A new drug in the treatment of sleeping sickness

Eflornithine (alpha-difluoromethylornithine; DFMO) is a recently developed drug against African trypanosomiasis (sleeping sickness). After a short description of trypanosomiasis and the current treatment, the mechanism of action of eflornithine is discussed, some clinical data is given and attention is paid to recently discovered analogues of eflornithine. Some examples of combination therapy with eflornithine and possible applications are mentioned.

High-pressure liquid chromatographic analysis of eflornithine in serum

Eflornithine [2-(difluoromethyl)-DL-ornithine monohydrochloride, monohydrate; DFMO] is an ornithine decarboxylase inhibitor used to treat Pneumocystis carinii pneumonia (PCP) in AIDS patients unresponsive or intolerant to conventional drug therapy. A reversed-phase HPLC method utilizing precolumn dansylation is described which permits the analysis of DFMO in serum of AIDS patients in support of pharmacokinetic studies. Norvaline (2-amino valeric acid) was added as the internal standard and the sample was extracted with three portions of ice-cold 80% ethanol. The supernatants were evaporated in a vacuum oven at 50 degrees C, and the residue was dissolved in base and derivatized at 25 degrees C for 4 h in the dark using dansyl chloride in acetone. Dilute base was then added and 25 to 50 microL was injected into the HPLC. Chromatography was performed on a C-8, 15-cm column with a linear gradient from a 95:5 solution of 10 mM pH 4 acetate buffer and THF, to a 90:10 solution of acetonitrile and THF over a period of 28 min. Detection was by UV at 330 nm, and DFMO and norvaline eluted at 14.7 and 17.7 min, respectively. The method was linear over a range of 5 to 950 micrograms/mL, and replicate analyses of a 25-micrograms/mL control specimen produced a between-run coefficient of variation of 6%. No interferences were encountered in a variety of patient and normal serum blanks.

Electrochemistry of the anticancer agents methotrexate and alpha-difluoromethylornithine in iminium form

The electrochemical characteristics of the antitumor agents methotrexate and alpha-difluoromethylornithine were determined as their iminium derivatives. Iminium formation from methotrexate is accomplished in vivo via protonation by enzyme. The requisite imine precursor is generated from alpha-difluoromethylornithine by condensation with enzyme containing pyridoxal phosphate. Electroreduction occurs in the range of -0.2 to -0.6 V. The relationship of reduction to structure is discussed. A possible mode of anticancer action involving electron transfer is presented.

Cationic antitrypanosomal and other antimicrobial agents in the therapy of experimental Pneumocystis carinii pneumonia

Cationic compounds used in the treatment of veterinary African trypanosomiasis have structural properties similar to those of pentamidine, which has been used in the therapy of human trypanosomiasis and infection with Pneumocystis carinii. We have compared the activities of these drugs and other antimicrobial agents in an immunosuppressed rat model of P. carinii pneumonia. Diminazene, imidocarb, amicarbalide, quinapyramine, and isometamidium showed efficacy greater than or equal to that of pentamidine in the therapy of P. carinii infection, whereas ethidium and methylglyoxal bis(guanylhydrazone) were only slightly active against the organism. Diminazene and pentamidine also exhibited comparable efficacy in P. carinii prophylaxis, alpha-Difluoromethylornithine (DFMO), a polyamine inhibitor, was ineffective therapy when used alone and did not improve the effectiveness of pentamidine or diminazene. Quinine, quinidine, quinacrine, chlorpromazine, spiramycin, Pentostam, Astiban, dehydroemetine, ampicillin, gentamicin, chloramphenicol, and spectinomycin also showed little or no activity against the organism. Thus, in this model anti-P. carinii activity appears to be a common property of veterinary cationic trypanocidal compounds. This should be important in studying structure-activity relationships and in developing new drugs for the treatment of P. carinii infection in humans.

Effects of alpha-difluoromethylornithine on protein synthesis and synthesis of the variant-specific glycoprotein (VSG) in Trypanosoma brucei brucei

Protein synthesis in Trypanosoma brucei brucei was rapidly inhibited during polyamine depletion by DL-alpha-difluoromethylornithine (DFMO) in vitro and in vivo. [3H]Leucine incorporation was depressed 30-40% by 24 h and 80-90% by 48 h of DFMO treatment. Concomitantly there was an apparent decrease in the synthesis of the variant-specific glycoprotein (VSG) in DFMO-treated trypanosomes, as measured by decreased incorporation of [3H]myristic acid into VSG. The discovery of decreased protein synthesis in T. b. brucei during DFMO treatment is noteworthy, because it was reported previously that protein synthesis was paradoxically stimulated 2-4-fold during DFMO treatment in these organisms. Decreased protein synthesis probably relates to the biochemical mechanism of action of DFMO on trypanosomes.

Synergism between 9-deazainosine and DL-alpha-difluoromethylornithine in treatment of experimental African trypanosomiasis

Kinetoplastid hemoflagellates are sensitive to growth inhibition by various purine analogs. In this study the activities of 9-deazainosine (9-DINO), formycin B, and sinefungin were compared in experimental murine Trypanosoma brucei subsp. brucei infections, both singly and in combination with the ornithine decarboxylase inhibitor DL-alpha-difluoromethylornithine (DFMO, eflornithine). Used singly, all of the purine analogs were able to suppress an acute T. brucei subsp. brucei infection. 9-DINO and formycin B were the most active. None of the purine analogs was curative when used singly against a strain causing chronic central nervous system infection. 9-DINO was highly effective when used in combination with DFMO in curing this central nervous system infection and another more stringent experimental infection. Neither sinefungin nor formycin B was active in combination with DFMO in curing the central nervous system experimental infection. 9-DINO was metabolized to phosphorylated derivatives of 9-deazaadenosine and 9-deazaguanosine by bloodstream trypomastigotes, but not by murine erythrocyte suspensions or kidney or liver homogenates--a potential rationale for the selectivity of the analog. These studies indicate that 9-DINO is a potent, nontoxic purine analog which, in combination with DFMO, is capable of late-stage cures of African trypanosomiasis.