Bone marrow, purines

Bone marrow aspirate supernatants have an approximately 10-fold greater concentration of purines than do simultaneous peripheral venous samples (Table 1)- AR comprises only 10 - 20% of bone marrow purines with Hx + X predominating. 

Adenosine deaminase deficiency is associated with an immunodeficiency disease in which both thymus-derived lymphocytes (T cells) and bone marrow-derived lymphocytes (B cells) are sparse and dysfunctional. Purine nucleoside phosphorylase deficiency is associated with a severe deficiency of T cells but apparently normal B cell function. Immune dysfunctions appear to result from accumulation of dGTP and dATP, which inhibit ribonucleotide reductase and thereby deplete cells of DNA precursors. 

Fig. 14.1 6-MP metabolism in bone marrow cells. 6-TIMP, 6-thioinosine monophosphate 6-mMP, 6-methylmercapto-purine 6-mTIMP, 6-methyl-thioinosine monophosphate.

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)... 

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... 

Thioguanine is a purine analog that has been used as an alternative treatment for psoriasis when conventional therapies have failed. The typical dose is 80 mg twice weekly, increased by 20 mg every 2 to 4 weeks the maximum dose is 160 mg three times a week. Adverse effects include bone marrow suppression, GI complications (e.g., nausea, diarrhea), and elevation of liver fimction tests. 6-Thioguanine may be less hepatotoxic and therefore more useful than methotrexate in hepatically compromised patients with severe psoriasis. 

Kerstens, P. J., Stolk, J. N., De Abreu, R. A., Lambooy, L. H., van de Putte, L. B., and Boerbooms, A. A. (1995) Azathiopiine-related bone marrow toxicity and low activities of purine enzymes in patients with rheumatoid arthritis. Arthritis and Rheumatism. 38, 142-145. 

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... 

Pemetrexed is chemically similar to folic acid. It inhibits three enzymes used in purine and pyrimidine synthesis - thymidylate synthetase, dihydrofolate reductase, and glycinamide ribonucleotide formyl transferase. By inhibiting the formation of precursor purine and pyrimidine nucleotides, pemetrexed prevents the formation of DNA and RNA. In 2004 it was approved for treatment of malignant pleural mesothelioma and as a second-line agent for the treatment of non-small cell lung cancer. Adverse effects include gastrointestinal complaints, bone marrow suppression, alopecia, allergic and neurotoxic reactions. 

Mycophenolate mofetil is used together with cyclosporine and corticosteroids for the prophylaxis of acute organ rejection in patients undergoing allogeneic renal, or hepatic transplants. Compared with azathioprine it is more lymphocyte-specific and is associated with less bone marrow suppression, fewer opportunistic infections and lower incidence of acute rejection. More recently, the salt mycophenolate sodium has also been introduced. Mycophenolate mofetil is rapidly hydrolyzed to mycopheno-lic acid, its active metabolite. Mycophenolic acid is a reversible noncompetitive inhibitor of inosine monophosphate dehydrogenase, an important enzyme for the de novo synthesis of purines. As lymphocytes have little or no salvage pathway for purine... 

Anti metabolites are structural analogues of normal cellular constituents that interfere with the synthesis of purine and pyrimidine, which are essential for DNA synthesis and cell division. They are used in the treatment of gastrointestinal and pulmonary carcinomas as well as sarcomas and some leukaemias. The major toxic effects are bone marrow suppression, mucositis, severe diarrhoea, nausea and vomiting. They have been associated with acute and chronic hepatotoxicity. 

Azathioprine is a cytotoxic inhibitor of purine synthesis effective for the control of tissue rejection in organ transplantation. It is also used in the treatment of autoimmune diseases. Its biologically active metabolite, mercaptopurine, is an inhibitor of DNA synthesis. Mercaptopurine undergoes further metabolism to the active antitumour and immunosuppressive thioinosinic acid. This inhibits the conversion of purines to the corresponding phosphoribosyl-5 phosphates and hypoxanthine to inosinic acid, leading to inhibition of cell division and this is the mechanism of the immunosuppression by azathioprine and mercaptopurine. Humans are more sensitive than other species to the toxic effects of the thiopurines, in particular those involving the haematopoietic system. The major limiting toxicity of the thiopurines is bone marrow suppression, with leucopenia and thrombocytopenia. Liver toxicity is another common toxic effect. 

Trimethoprim- sulfamethoxazole Synergistic combination of folate antagonists blocks purine production and nucleic acid synthesis Bactericidal activity against susceptible bacteria Urinary tract infections Pneumocystis jiroveci pneumonia toxoplasmosis nocardiosis Oral, IV renal clearance (half-life 8 h) dosed every 8-12 h t formulated in a 5 1 ratio of sulfamethoxazole to trimethoprim Toxicity Rash, fever, bone marrow suppression, hyperkalemia... 

The effects of cobalamin deficiency are most pronounced in rapidy dividing cells, such as the erythropoietic tissue of bone marrow and the mucosal cells of the intestine. Such tissues need both Die N5-N10-methylene and N10-formyl forms of tetrahydrofolate for Ihe synthesis of nucleotides required for DNA replication (see pp. 291, 301). However, in vitamin B12 deficiency, the N5-methyl form of tetrahydrofolate is not efficiently used. Because the methylated fonn cannot be converted directly to other forms of tetrahydrofolate, tie Ns-methyl form accumulates, whereas the levels of the other forms decrease. Thus, cobalamin deficiency is hypothesized to lead to a deficiency of the tetrahydrofolate forms needed in purine and thymine synthesis, resulting in the symptoms of megaloblastic anemia. 

Besides this salvage role, hypoxanthine-guanine phosphoribosyltransferase is probably important also for the transfer of purines from liver to other tissues. Purine biosynthesis de novo is especially active in the liver, and extrahe-patic cells that have a low capacity for the synthesis of purines de novo, such as erythrocytes and bone marrow cells, depend on uptake of hypoxanthine and xanthine from the... 

T There are several distinct types of inhibitors of nucleotide biosynthesis, each type acting at different points in the pathways to purine or pyrimidine nucleotides. All these inhibitors are very toxic to cells, especially rapidly growing cells, such as those of tumors or bacteria, because interruption of the supply of nucleotides seriously limits the cell s capacity to synthesize the nucleic acids necessary for protein synthesis and cell replication. In some cases, the toxic effect of such inhibitors makes them useful in cancer chemotherapy or in the treatment of bacterial infections. However, some of these agents can also damage the rapidly replicating cells of the intestinal tract and bone marrow. This danger imposes limits on the doses that can be used safely. 

Methotrexate acts by inhibition of dihydrofolate reductase, the enzyme requisite for the reduction of dihydrofolic acid (3) to 5,6,7,8-tetrahydrofolic acid (4). In turn, (4) is a precursor to a series of enzyme cofactors (5-7) essential for the transfer of one carbon unit necessary for the biosynthesis of purines and pyrimidines and hence, ultimately, DNA. As an inhibitor of dihydrofolate reductase, methotrexate kills cells during the S phase of the cell cycle, when the cells are in the log phase of growth. Unfortunately, this cytotoxicity is non-selective, and rapidly proliferating normal cells, e.g., gastrointestinal epithelium cells and bone marrow, are dramatically affected as well. In addition, recent use of high dose methotrexate therapy with leucovorin rescue has led to additional clinical problems arising from a dose-related nephrotoxic metabolite, 7-hydroxy methotrexate (8). Finally, the very polar nature of methotrexate renders it virtually impenetrable to the blood-brain barrier, which can necessitate direct intrathecal injection in order to achieve therapeutic doses for the treatment of CNS tumours. 

Cytotoxic agents destroy immimologically competent cells. Azathioprine, a prodrug for the purine antagonist mercaptopurine, is used in autoimmune disease because it provides enhanced immunosuppressive activity. Cyclophosphamide is a second choice. Bone marrow is depressed as is to be expected. 

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