Purine analogues

The mode of action has been a subject for research for a number of years. While it was originally thought that maleic hydrazide replaced uracil in the RNA sequence, it has been deterrnined that the molecule may be a pyrimidine or purine analogue and therefore base-pair formation is possible with uracil and thymine and there exists the probabiHty of base-pair formation with adenine however, if maleic hydrazide occurs in an in vivo system as the diketo species, then there remains the possibiHty of base-pairing with guanine (50). Whatever the mechanism, it is apparent that the inhibitory effects are the result of a shutdown of the de novo synthesis of protein. 

Czochralska, B., Wrona, M., and Shugar, D. Electrochemically Reduced Photorevcrsible Products of Pyrimidine and Purine Analogues. 130, 133-181 (1985). 

Balzarini J, Velazquez S, San-Felix A, Karlsson A, Perez-Perez M-J, Camarasa M-J, De Clercq E. Human immunodeficiency virus type 1-specific [2, 5 -bis-0-(tert-butyldimethylsilyl)-(i-D-ribofuranosyl]-3 -spiro-5"-(4"-ami-no-1",2"- oxathiole-2",2"-dioxide)-purine analogues show a resistance spectrum that is different from that of the human immunodeficiency virus type 1 -specific non-nucleoside analogues. Mol Pharmacol 1993 43 109-114. 

Cladribine (2-CdA) -purine analogue antimetabolite -bone marrow suppression -fever in 50% (probably due to tumor lysis) -rash in 50% -immunosuppression (with profound T-cell lymphopenia)... 

CD Fludarabine -purine analogue antimetabolite partially cell cycle specific... 

Mercapto- - purine analogue antimetabolite predominantly S-phase ... 

Pentostatin -purine analogue antimetabolite inhibits adenosine deaminase -nephrotoxicity (including acute renal failure) -bone marrow suppression -neurotoxicity—lethargy, fatigue, seizures, coma -immunosuppression (lymphopenia) -nausea and vomiting -fever -anorexia -hepatotoxicity... 

Thioguanine (6-TG) -purine analogue antimetabolite cell cycle dependent -bone marrow suppression -nausea and vomiting -mucocutaneous effects (mucositis, stomatitis) -rash -hepatotoxicity -hyperuricemia... 

V79 System. The Chinese hamster V79 line was established in 1958 (Ford and Yerganian, 1958). Publication of the use of the line for mutation studies (by measuring resistance to purine analogues due to mutation of the Hgprt locus) occurred 10 years later (Chu and Mailing, 1968). The V79 line was derived from a male Chinese hamster hence, V79 cells possess only a single X chromosome. 

Chlorocyclononatetraene 267 rapidly rearranges in liquid sulfur dioxide to 1-chloro-8,9-dihydroindene 268, which forms the cycloadduct 269 with the triazolinedione 264 (equation 145)137. The vinylimidazole 270 affords the purine analogue 271 (equation 146)138. 

Specific structural features also determine the deactivation or destruction by catabolic enzymes of purine analogues. Selective destruction of these toxic materials may, in fact, be the sin e most important factor relating to their activity. 

In the sections that follow, the metabolism of the various purine analogues is presented first, followed by a discussion of the current status of our knowledge about the mechanism of action of these compounds. Next the distribution and effects of these drugs on the host and invading organisms is considered with some concluding remarks on the ever present problem of resistance. 

Little quantitative work has appeared on the determination of the rate of conversion of the various purine analogues to their nucleotides with highly purified phosphoribosyltransferases from mammalian cells or any other source, but there would appear to be a rough correlation between the cytotoxicity of these analogues and their ability to serve as substrates (the Kj values for a number of purines and purine analogues have been determined [45a, 85a] but this value is not a measure of conversion to nucleotide). Table 2.1 lists a number of purine analogues and an estimate of their ability to serve as substrates for the phosphoribosyltransferases. 

In addition to the enzymes that catalyse the formation of nucleotides and polynucleotides, a large number of catabolic systems exist which operate at all levels of the internucleotide pathways. The ribonucleases and deoxyribonucleases that degrade polynucleotides are probably not significantly involved in purine analogue metabolism, but the enzymes which dephosphorylate nucleoside 5 -monophosphates are known to attack analogue nucleotides and may be of some importance to their in vivo activity. Phosphatases of low specificity are abundant in many tissues [38], particularly the intestine [29]. Purified mammalian 5-nucleotidases hydrolyse only the nucleoside 5 monophosphates [28] and... 

The literature on xanthine oxidase [84] and its companion catabolic enzyme uricase [87] has been extensively reviewed. Many purine analogues, with the exception of most 9-substituted purines [262], serve as substrates for xanthine oxidase both in vitro and in vivo, and if the product is a substrate for uricase, in species that possess this enzyme, the ultimate product is allantoin (LVIII). Thus 2-aminoadenine [5], A -methyladenine [122], and purine [129] are all catabolized... 

Table 2.3. CORRELATION OI THE CYTOTOXICITY OF CERTAIN PURINE ANALOGUES AND THEIR RIBONUCLEOSIDES WITH THEIR INHIBITION OF FGAR...

A comparison of EDso values for feedback inhibition and for growth inhibition in H.Ep.-2 cells in culture is shown in Table 2.3. It is readily apparent that for most of the purine analogues listed, the correlation between feedback inhibition and cytotoxicity is good. The few discrepancies may be due to the fact that these particular compounds are not feedback inhibitors, but metabolism (or lack of it) of the analogue in question may be important. In comparing these ED50 values, the difference in experimental conditions for cytotoxicity determination (long... 

The subject of the incorporation of anticancer agents into macromolecules [13] and other compounds [336] has been reviewed. A number of purine analogues are incorporated into nucleic acid, but the incorporation of these compounds requires that they be anabolized to nucleoside mono-, di-, and triphosphates, and it is difficult to separate the metabolic effects of the nucleoside phosphates from the metabolic effects of the fraudulent polynucleotides. 

Azaguanine was the first purine analogue shown to be incorporated into polynucleotides [337] and, since its primary metabolic effect is on protein synthesis, the incorporation into RNA is considered the basis for its biologic activity [338]. In microbial systems 8-aza-adenine, 8-azahypoxanthine, 8-azaxanthine, and 5(4)-amino-l/f-l, 2, 3-triazole-4(5)-carboxamide are all incorporated into RNA as 8-azaguanylic acid [336]. 

Thus there are a number of metabolic events that are interfered with by the various purine analogues. The biological consequences of these interferences are discussed in the sections that follow. 

Studies on the toxicology of some of these purine analogues in rodents and dogs at sublethal doses showed that rapidly dividing tissues, especially the intestinal mucosa and bone marrow, are most sensitive to these compounds... 

Table 2.4. A COMPARISON OF THE TOXICITY OF SOME PURINE ANALOGUES...

The finding that the administration of 6-mercaptopurine to rabbits following exposure to bovine serum albumin prevented antibody formation [374] formed the basis for a new area of chemotherapy for purine analogues and other antimetabolites and was soon followed by the use of these drugs for the therapy of autoimmune disease and the suppression of homograft rejection. This subject has been reviewed in depth [ 12, 375, 375a], has occasioned a symposium [376], and has received much recent publicity as a result of human heart transplants. 

Much information on the mechanism of action and cross-resistance of purine analogues has been obtained in bacteria, some of which are quite sensitive to certain of these compounds in vitro. There is a great deal of variation in response of the various bacteria to a particular agent and of a particular bacterium to the various cytotoxic purine analogues. Some, if not most, of these differences are probably due to differences in the anabolism of the various compounds. Despite the fact that certain purine analogues have quite a spectrum of antibacterial activity in vitro, none has been useful in the treatment of bacterial infections in vivo because their toxicity is not selective—the metabolic events whose blockade is responsible for their antibacterial activity are also blocked in mammalian cells and thus inhibition of bacterial growth can only be attained at the cost of prohibitive host toxicity. In contrast, the sulpha drugs and antibiotics such as penicillin act on metabolic events peculiar to bacteria. 

It is of historical interest that Tetrahymena gelii, whose metabolism has been described in detail [387], is inhibited by 8-azaguanine [388] and other purine analogues [389, 390]. Of more importance to chemotherapy is the fact that pathogenic protozoa such as the trypanosomes respond in vitro to a number of... 

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