7- -8-mercaptopurines formation

For reasons discussed in Section VI, a survey of the purine series (29) is being made in this Department, but so far no example (including 2-hydroxy- and 8-trifluoromethyl-2-hydroxy-purine) of covalent hydration has come to light. An examination of ionization constants disclosed no apparent anomalies, although the interpretation is made more difficult by the ease of anion formation in the 9-position, which often competes with that from other anionic substituents. The only abnormal spectrum seems to be that of the anion of 2-mercaptopurine which is being further examined. 

The carbons added in reactions 4 and 5 of Figure 34-2 are contributed by derivatives of tetrahydrofolate. Purine deficiency states, which are rare in humans, generally reflect a deficiency of folic acid. Compounds that inhibit formation of tetrahydrofolates and therefore block purine synthesis have been used in cancer chemotherapy. Inhibitory compounds and the reactions they inhibit include azaserine (reaction 5, Figure 34—2), diazanorleucine (reaction 2), 6-mercaptopurine (reactions 13 and 14), and mycophenofic acid (reaction 14). 

I. Y. Hwang, A. A. Elfarra, Cysteine S-Conjugates May Act as Kidney-Selective Prodrugs Formation of 6-Mercaptopurine by the Renal Metabolism of, V-(6-Purinyl)-i,-cysteine , J. Pharmacol. Exp. Ther. 1989, 251, 448 - 454. 

Cysteine-S-conjugates have also been proposed as kidney-selective pro-drugs. Renal metabolism of S-6-(purinyl)-L-cysteine resulted in the formation of 6-mercaptopurine by the action of P-lyase [51]. However, besides formation of the intended parent compound, other S-conjugates may be formed by various radical reactions, which may induce renal toxicity. 

The effect of 6-mercaptopurine on the incorporation of a number of C-labelled compounds into soluble purine nucleotides and into RNA and DNA has been studied in leukemia L1210, Ehrlich ascites carcinoma, and solid sarcoma 180. At a level of 6-mercaptopurine that markedly inhibited the incorporation of formate and glycine, the utilization of adenine or 2-aminoadenine was not affected. There was no inhibition of the incorporation of 5(or 4)-aminoimidazole-4(5)-carboxamide (AIC) into adenine derivatives and no marked or consistent inhibition of its incorporation into guanine derivatives. The conversion of AIC to purines in ascites cells was not inhibited at levels of 6-mercaptopurine 8-20 times those that produced 50 per cent or greater inhibition of de novo synthesis [292]. Furthermore, AIC reverses the inhibition of growth of S180 cells (AH/5) in culture by 6-mercaptopurine [293]. These results suggest that in all these systems, in vitro and in vivo, the principal site at which 6-mercaptopurine inhibits nucleic acid biosynthesis is prior to the formation of AIC, and that the interconversion of purine ribonucleotides (see below) is not the primary site of action [292]. Presumably, this early step is the conversion of PRPP to 5-phosphoribosylamine inhibited allosterically by 6-mercaptopurine ribonucleotide (feedback inhibition is not observed in cells that cannot convert 6-mercaptopurine to its ribonucleotide [244]. 

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. 

The coenzyme tetrahydrofolate (THF) is the main agent by which Ci fragments are transferred in the metabolism. THF can bind this type of group in various oxidation states and pass it on (see p. 108). In addition, there is activated methyl, in the form of S-adenosyl methionine (SAM). SAM is involved in many methylation reactions—e. g., in creatine synthesis (see p. 336), the conversion of norepinephrine into epinephrine (see p. 352), the inactivation of norepinephrine by methylation of a phenolic OH group (see p. 316), and in the formation of the active form of the cytostatic drug 6-mercaptopurine (see p. 402). 

A one-pot procedure for the transformation of 6-thiopurine nucleosides to 6-aminopurines was developed using DMDO as the oxidant in the presence of a stoichiometric amount of various amines <1996T6759>. For example, 6-thio-9-(2, 3, 5 -tri-0-acetyl-/3-D-ribosyl)purine was readily converted to the 6-alkylamino derivatives (6-amino, 75% yield 6-methylamino, 55% yield). Similarly, A -6-acetyl-8-thio-9-(2, 3, 5 -tri-0-acetyl-/3-D-ribosyl)adenosine was converted to A -6-acetyl-8-methylamino-9-(2, 3, 5 -tri-0-acetyl-/3-D-ribosyl)adenosine (DMDO, methylamine, CH2CI2, 25 °C, 83% yield). Less nucleophilic 2-mercaptopurine derivatives did not undergo the displacement reaction, however, and only the products of dithiane formation and desulfurization were isolated. 

Purine Antimetabolites. Purine synthesis can be blocked by 6-mercaptopurine (7.77) and 6-thioguanine (7.78). Both require conversion to the mononucleotide in a lethal synthesis —a mechanism distinguished from the formation of suicide substrates in that the enzyme that transforms the inactive pro-dmg to the active inhibitor is different from the enzyme that is being blocked. inhibitors are formed and bound by the same... 

Mercaptopurine (6MP) and 6-thioguanine (6TG) are analogs of the purines hypoxanthine and guanine, which must be activated by nucleotide formation, according to the following scheme ... 

Enzyme assays were conducted in a 10 mL screw-neck glass test tube containing 100 fiL of lysate, 90 fiL of a 250 /ng/mL solution of 6-mercaptopurine in 0.01 M HC1, and 15 /uL of 250 mM sodium phosphate buffer (pH 9.2). Reactions were initiated by the addition of 32 fiL of a 3 1 mixture of 250 fiM S-adenosyl-L-methionine and 30 mM dithiothreitol. The final pH was 7.5. After a 1-hour incubation at 37°C, the reaction was stopped by the addition of 850 fjL of ice-cold 3.5 mM dithiothreitol and 50 fih of 1.5 M H2S04. The tubes were then heated at 100°C for 2 hours. To each tube, 500 fiL of 3.4 M NaOH was added, immediately followed by 8 mL of toluene-amyl alcohol-phenyl mercuric acetate. The tubes were shaken for 10 minutes and centrifuged. Then 6 mL of the toluene layer was transferred to a glass-stoppered conical test tube and 0.2 mL of 0.1 M HC1 added. After vortex-mixing and centrifuging, the toluene layer was discarded. Samples (50 fiL in 0.1 M HC1) were used for HPLC analysis. Product formation was linear for up to 120 minutes and 150 /u,L of lysate. 

The evidence of the effect of complexation was established by the absence of an absorption band at 3191 cm (N-H stretching characteristic of a purine function in Azathioprine) e.g., the aminogroup is involved in the formation of the metal complex. The anticancer action of purine derivatives was discovered in 1949. This cytostatic drug biotransforms to 6-mercaptopurine in the body. ... 

The usual initial dose is 2 mg/kg daily hy the oral route. If there is no clinical improvement or leukopenia after 4 weeks the dosage is increased to 3 mg/kg daily. In contrast to mercaptopurine, thioguanine may be continued in the usual dose when allopurinol is used to inhibit uric acid formation. 

Since specificity of the nucleoside phosphorylases varies, enzymes that catalyze the formation of the nucleosides of the so-called fraudulent purine bases, 6-mercaptopurine (purine-6(lH)-thione) and 8-aza-guanine (5-amino-t -triazolo[ 4,5-d]pyrimidin-7(6if)-one), have been found, and gram quantities of 8-azaguanosine have been isolated. ... 

Thiols are often oxidized to disulfides via a photoinitiated radical path, as with thiorfan (111, Scheme 4.67) (Gimenez et al., 1988), or up to sulfinate and sulfonate as with the antineoplastic 6-mercaptopurine (Hemmens and Moore, 1984). Both alkyl and aryl sulfides are photooxidized to sulfoxides, as in the formation of S-oxides (110) from dibenzothiazepin (109, Scheme 4.66), as well as (21) from chlorpromazine (20, Scheme 4.16) and similarly from thioridazine (Elisei et al., 2002) and diltiazem (Andrisano et al., 2001b). 

Include GI distress, peripheral neuropathy, rash, vasculitis, and stone formation. Inhibits 6-mercaptopurine (o-MP) metabolism. 

Allopurinol is a uricosuric drug used in chronic gout that prevents formation of uric acid from purines by acting as a suicide substrate of xanthine oxidase. The drug is commonly used in patients undergoing treatment of cancer to slow down formation of uric acid derived from purines released by the cytotoxic action of drugs or radiation. The metabolism of 6-mercaptopurine (6-MP), a substrate for xanthine oxidase, is also inhibited by allopurinol, necessitating a major dose reduction to avoid its toxic effects. 

Mercaptopurine is a purine analogue that substitutes for purine bases (adenine and guanine) in the synthesis of DNA. It is converted to a false nucleotide that inhibits the formation of normal DNA. 

An interesting example of a DDI due to the inhibition of a non-CYP enzyme that can have serious clinical consequences is the inhibition of xanthine oxidase by allopu-rinol 6-mercaptopurine (6-MP) as an antimetabolite type of antineoplastic drug. One of its indications is in the treatment of inflammatory bowel disease. Actually, 6-MP is a prodrug whose active metabolite, 6-thiogua-nine (6-TG) is responsible for its therapeutic activity. Some nonresponders to 6-MP do not form sufficient amounts of 6-TG. A complementary pathway of 6-MP metabolism is oxidation to 6-thiouric acid (6TU), which is mediated by xanthine oxidase. Inhibition of this complementary pathway by allopurinol shunts the metabolism of 6-MP favoring increased formation of 6-TG. 

Cell proliferation may be decreased by using various structural analogues to inhibit nucleic acid synthesis. Amethopterin (methotrexate) closely resembles folic acid (page 165) in structure and prevents the formation of the purine precursors of DNA, RNA and ATP. 6-Mercaptopurine is a purine analogue which interferes with the pathway in which the phosphoribosyl precursors of the purine nucleotides are formed. 5-Fluorouracilis a competitive inhibitor of the conversion of uridylic acid (dUMP) into thymidylic acid (dTMP). 

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