[CANCER RESEARCH 44, 3144-3148. July 1984]

Hypoxanthine Concentrations in Normal Subjects and Patients with Solid Tumors and Leukemia1

Wally E. Wung and Stephen B. Howell2

Department of Medicine, the Cancer Center, and the General Clinical Research Center, University of California, San Diego, La Jolla, California 92093

creased plasma Hyp3 concentrations during treatment may also ABSTRACT be a mechanism of resistance to antimetabolites acting in the Mean plasma hypoxanthine (Hyp) concentrations determined purine biosynthetic pathways. Extracellular Hyp is important for by high-pressure liquid chromatography were 0.56 MM (range, the activity of purine antimetabolites in 2 aspects: it can be 0.2 to 1.9 MM)in 16 normal subjects, 0.68 MM(range, 0.1 to 1.1 utilized to restore purine nucleotide pools in cells the de novo MM)in 10 untreated acute leukemic subjects, and 0.89 MM(range, synthesis of which has been blocked, and it potentially may 0.3 to 2.6 MM)in 14 solid tumor patients. Despite large differences compete with some of these antimetabolites for transport into in Hyp concentration between patients, every 4-hr sampling, the cell or for the enzymes necessary to convert the drugs to indicated that diurnal variation in individual patients was small their active nucleotides (9). (maximum, 2.3-fold). While the mean plasma and malignant Using a newly developed high-pressure liquid Chromatographie effusion Hyp concentrations did not differ significantly, bone method (43), we have measured plasma Hyp in groups of normal marrow plasma Hyp concentration averaged 4.0-fold greater subjects, patients with solid tumors, and patients with acute than that of simultaneously drawn venous plasma. Allopurinol leukemia. Frequent blood samples were obtained from some 300 mg p.o. caused a mean 1.5-fold increase in plasma Hyp patients to assess diurnal variation, and Hyp concentration in within 3 hr. In 17 patients with acute leukemia, treatment with bone marrow and malignant effusions was compared to that of allopurinol at 300 mg daily plus initiation of chemotherapy caused venous plasma to determine the distribution of Hyp in different a mean 7-fdd increase in plasma Hyp to 4.6 MM(range, 1 to 12 body compartments. We also examined the effect on plasma MM).The ability of Hyp to modulate the toxicity of antimetabolites Hyp of treatment with HPP alone or the combination of HPP and affecting purine synthesis (6-diazao-5-oxonorleucine, 6-methyl- chemotherapy. Finally, the effect of Hyp at concentrations en mercaptopurine riboside, 6-mercaptopurine, and 6-thioguanine) compassing the range observed in vivo was tested for its ability was determined in vitro using human B-lymphoblast (WI-L2) and to modulate the toxicity of de novo purine synthesis inhibitors promyelocytic leukemia (HL-60) cell lines. Hyp permitted growth toward the HL-60 promyelocytic leukemia and the WI-L2 lym- of both cell lines in the presence of clinically achievable concen phoblast human cell lines. trations of all 4 drugs, but the initial culture concentrations of Hyp required were above those found in patients. Since Hyp MATERIALS AND METHODS was consumed rapidly during the culture period, the average Hyp concentrations required for the protection of cells were Subjects and Patients. Blood samples were obtained from 5 groups actually much lower. We conclude that, in patients with acute of subjects and patients for Hyp measurement: (a) normal laboratory personnel; (b) patients with acute lymphocytic leukemia or acute nonlym- leukemia receiving allopurinol during chemotherapy, plasma Hyp phocytic leukemia, either untreated or in relapse; (c) patients with ad concentrations are significantly elevated; the potential for antag vanced solid tumors of many types; (d) patients with malignant pleural onism of antimetabolite activity is uncertain. or peritoneal effusions due to solid tumor involvement; and (e) patients with acute leukemia undergoing initial induction chemotherapy for either INTRODUCTION untreated or relapsed disease. No restrictions were placed on diet or Antimetabolites which interfere with de novo purine synthesis activity in any group of subjects or patients, and samples were drawn at constitute an important part of the armamentarium of drugs random times of day. For all groups except the normal subjects and acute leukemic subjects receiving induction chemotherapy, blood sam available for the treatment of leukemia and other cancers (4, 15, ples were obtained at least 3 weeks following any prior therapy and at a 26, 36). However, not all patients respond equally well to these time when no toxicity attributable to prior therapy was evident. drugs, and some eventually become refractory to the therapy. High-Pressure Liquid Chromatography. A detailed procedure for Resistance to purine analogues has been attributed to the defi simultaneous measurement of plasma Hyp, uric acid, HPP, and oxipurinol ciency of enzymes necessary to convert them to their nucleotide was developed in this laboratory and has been published elsewhere (43). forms (7, 31). Other mechanisms have been observed in experi Delay in separation of plasma from the formed elements of blood has mental tumors, such as increased degradation of drugs and been shown to increase plasma Hyp. Therefore, samples were collected inability of resistant cells to convert the ribonucleotide analogues in prechilled heparinized tubes, and plasma was separated immediately to the deoxyribonucleotide analogue (10, 29). by centrifugation, precipitated with 0.1 volume of 4.4 N HCIO<, and This study was designed to examine the possibility that in- neutralized with Alamine/Freon, as described by Khym (21). A high- pressure liquid Chromatographie system (Waters Associates, Milford, 1This work was supported by Grants CA23100 and CA23334 from the National MA) with a reverse-phase C,fi-^Bondapak column was used in this study. Cancer Institute and by the University of California, San Diego General Clinical Peaks were identified by their retention times and their absorbance ratios Research Center, NIH, Division of Research Resource (Grant RR00827). This at 254 and 280 nm. An aliquot of 20-/il sample was analyzed isocratically research was conducted in part by the Clayton Foundation for Research, California Division. 'The abbreviations used are: Hyp, hypoxanthine; DON, 6-diazo-5-oxonorteu- 2 Clayton Foundation Investigator. To whom requests for reprints should be cine; HPP, 4-hydroxy-3,4-pyrazotopyrimidine (allopurinol); HGPRT, hypoxanthine- addressed. guanine phosphoribosyltransferase; 6-MP, 6-mercaptopurine; MMPR, 6-methyl- Received April 10, 1981 ¡accepted April 2, 1984. mercaptopurine riboside; 6-TG. 6-thioguanine.

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Downloaded from cancerres.aacrjournals.org on October 1, 2021. © 1984 American Association for Cancer Research. Plasma Hyp and Antimetabolite Activity using phosphate eluent (50 HIM KH2PO4, pH 4.60). The limit of detection for all compounds of interest was 0.1 MM. IO'6 Cell Culture and Growth Studies. A human B-lymphoblast (WI-L2) and a human promyelocytic leukemia (HL-60) cell line were used to determine the growth-inhibitory effect of various concentrations of anti- metabolites and the blockade of this effect by Hyp in vitro. These cells were grown in RPMI 1640 medium (Cell Culture Facility, University of California, San Diego, La Jolla, CA) supplemented with 10% fetal calf serum (Irvine Scientific, Irvine, CA), 1% glutamine, and 1% antibiotics- antimycotics mixture (Grand Island Biological Co., Grand Island, NY). Growth assays were performed in LJnbro multiwell tissue culture plates ia,-7 (Flow Laboratories, Inc., McLean, VA). The tog-phase cells were washed 0600 1200 1800 2400 with RPM11640 medium to remove any Hyp in the growth medium. Hyp TIME OF DAY and antimetabolites (6-MP, 6-TG, MMPR, and DON) were prepared in Chart 2. Diurnal variations of plasma Hyp concentration in 4 untreated solid tumor patients. No restrictions were placed on the patients' diets or activities. the same medium, except that fetal calf serum was replaced with bovine serum albumin (5 mg/ml) or 10% charcoal-treated fetal calf serum. Cells were cultured at 105/ml in triplicate with 1 to 100 MM Hyp and 1, 4, and patients was 410 MM(range, 110 to 610 MM)(data not shown), 10 MM of each antimetabolite at 37° for 3 days under 4% CO2. The which is 1.5-fold greater than the mean of 276 MM(range, 210 number of cells was counted electronically using a Model ZB Coulter Counter, and the percentage of control growth in antimetabolite-treated to 340 MM)found in the normal subjects; plasma uric acid in the cultures was calculated by comparison with cells growing in Hyp-free solid tumor patients was not significantly different from that in and antimetabolite-free medium. normal subjects (mean, 255 MM;range, 150 to 440 MM). Diurnal Variation of Plasma Hyp. Four solid tumor patients were studied for diurnal variation of plasma Hyp. None of these RESULTS patients was receiving chemotherapy or HPP at the time of Means and Ranges of Plasma Hyp. Previous reports have sampling. Blood was collected at intervals of 4 hr for a period of indicated that, in acute leukemia, both the production and plasma 24 hr. No restriction was placed on food intake or physical concentration of uric acid were increased (29). Similar changes activity during the sampling time. Chart 2 shows that, in each of in plasma Hyp, which is the precursor of uric acid, have not been these 4 patients, plasma Hyp concentration remained within reported. A comparison of plasma Hyp concentrations in ran relatively narrow limits over 24 hr compared to the wider range domly collected blood samples from 16 normal subjects, 10 found between patients. The average variation in plasma Hyp concentration over 24 hr was 1.9-fold, and the range was 1.4- untreated acute leukemic subjects, and 14 untreated solid tumor to 2.3-fold. The variation could not be related to meals, activity, patients is summarized in Chart 1. The geometric mean plasma Hyp concentration was 0.56 MM(range, 0.2 to 1.9 MM)in normal or sleep. subjects, 0.68 MM (range, 0.1 to 1.1 MM) in acute leukemic Effects of HPP and Chemotherapy on Plasma Hyp. The subjects, and 0.89 MM (range, 0.3 to 2.6 MM) in solid tumor effect of a single dose of 300 mg of HPP p.o. on plasma Hyp patients, and the differences between these groups were not concentration and the pharmacokinetics of HPP were examined significant. Included in Chart 1 are measurements made on in the same 4 patients in whom diurnal variation was also studied. malignant ascites or pleural effusions from solid tumor patients. Chart 3 shows the plasma concentrations of Hyp, HPP, and The geometric mean effusion Hyp concentration was 0.4 MM oxypurinol, the major metabolite of HPP, during the 8 hr following (range, 0.1 to 1.0 MM),about one-half of that in plasma of solid HPP ingestion. In 3 of 4 patients, plasma HPP peaked at 2 hr and then fell rapidly, with an elimination half-life of 55 to 65 min. tumor patients. Mean plasma uric acid concentration in these acute leukemic In the fourth patient, gastrointestinal absorption of HPP was 4xlO~~ delayed. Maximum plasma oxypurinol was reached about 2 hr after HPP ingestion, and it averaged 35 MM(range, 21 to 60 MM). This level remained steady for each patient over the next 6 hr. Other investigators have also reported a long half-life of 14 to g 5 20 hr for oxypurinol (3, 23). Plasma Hyp concentration increased ft to a mean of 1.35 MM(range, 0.55 to 2.50 MM)during the 6-hr ,-6 IxlO period following HPP ingestion. There was a 1.5-fold increase T over the mean Hyp concentration during the preceding 24 hr before the administration of HPP. Plasma Hyp was measured before and then again 48 hr after the initiation of treatment with HPP, daunorubicin, and 1-ß-o- arabinofuranosylcytosine on 6 courses in 5 patients with acute o. leukemia (Chart 4). In addition, individual nonpaired measure ments were made either before or 48 hr after initiation of treat ,-7 IxlO ment on 11 courses given to 6 other acute leukemic subjects. NORMAL LEUKEMIC SOLID MALIGNANT SUBJECTS PATIENTS TUMOR EFFUSIONS Whereas a single dose of HPP alone did not substantially in PATIENTS crease plasma Hyp, the combination of HPP plus the initiation of Chart 1. Scattergram of plasma Hyp concentrations in normal subjects (n = chemotherapy did. Considering only the 6 paired observations, 16), acute leukemic subjects (n - 10), and solid tumor patients (n = 14). Hyp the plasma Hyp concentration increased by a mean of 8.6-fold concentrations in malignant effusions (n = 9) are also included. Bars, geometric means. (p < 0.01 ; f test for paired observations). When all measurements

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Downloaded from cancerres.aacrjournals.org on October 1, 2021. © 1984 American Association for Cancer Research. IV.E. Wung and S. B. Howe// OXIPURINOL tions of Hyp in experimental cultures. 10-4r Chart 6 shows the effect of initial Hyp concentration between 1 and 100 ^M on the proliferation of WI-L2 and HL-60 cells in the presence of 6-MP, 6-TG, MMPR, and DON. Antimetabolite concentrations from 1 to 10 ^M were chosen as representative of the concentrations of these drugs that can be achieved clinically (25, 27). Increasing the Hyp concentration 10-fold from 1 to 10 pM resulted in increased growth at all 3 concentrations V* \ of all of the drugs; however, the magnitude of this effect was not large. A further 10-fold increase in initial Hyp concentration increased growth substantially, even in cultures treated with the highest antimetabolite concentrations, to 50% or more of the growth occurring in non-drug-treated control cultures for both cell lines. Thus, Hyp antagonized the antiproliferative effect of all 4 drugs, but a 50% reduction in antimetabolite effect required Hyp concentration above those observed even in patients with 01 2345678 acute leukemia receiving HPP and induction therapy who had a marked increase in Hyp production. It is important to emphasize that the data depicted in Chart 6 HYPOXANTHINE I0~5r

O

g I- z Lu O HOURS S 10-6 Chart 3. Plasma HPP ( ), oxypurinol ( ), and Hyp concentration time LU O curves obtained in the same patients as in Chart 2 following the p.o. ingestion of a ZJ 300-mg dose of HPP. l-X were included, the plasma Hyp concentration was 7-fold higher o_l at 24 hr, with a mean of 4.6 pM (range 0.9 to 12.0 /UM).A linear relationship was observed between plasma Hyp and oxypurinol concentrations measured in the same samples (r = 0.71 ; p < 0.01). IO',-7 Hyp Concentration in Bone Marrow. Hyp concentration in PRIOR TO 48HR AFTER bone marrow was compared with venous blood plasma in 9 solid TREATMENT START OF CHEMO tumor patients, and the results are shown in Chart 5. In all THERAPY AND HPP patients, the marrow plasma Hyp concentrations were consist Chart 4. Plasma Hyp concentrations in acute leukemic patients before and 48 hr after the administration of HPP and combined chemotherapy consisting of ently greater than those of simultaneously drawn venous plasma. doxorubicin and 1-/3-D-arabinofuranosylcytosine. HPP was given in daily doses of The mean difference between marrow plasma and venous 300 mg p.o. during chemotherapy. Measurements made on the same patients are plasma was 5.7 pu (p < 0.01 ; f test for paired observations). connected by a solid line; bars, geometric means. The possibility that this difference was due to hemolysis in marrow specimens was excluded by the finding that there was no increase in Hyp concentration in marrow plasma when RBC 1-5 were added and subjected to mechanical destruction. Effect of Hyp on Antimetabolite Activity. Both WI-L2 and HL-60 cells are normally grown in medium containing 10% fetal is calf serum, which itself contributes large amounts of Hyp to the culture. In order to avoid this problem, WI-L2 cells were grown in purine-free RPMI 1640 medium containing bovine serum al 1°•*-/* bumin (5 mg/ml), transferrin (35 pg/m\), insulin (5 pg/m\), 2.5 nw ,-6 selenium, and 20 pM ethanolamine. HL-60 cells were grown in 10

RPM11640 medium containing charcoal-stripped fetal calf serum PLASMA BONE MARROW at a final concentration of 10%. The fetal calf serum contained Chart 5. Comparison between venous plasma and bone marrow plasma Hyp less than 1 pM Hyp after treatment with charcoal (0.5 g per 10 concentrations in 9 solid tumor patients. Venous blood was collected at the moment ml of serum) as determined by high-pressure liquid chromatog- of bone marrow aspiration; plasma was recovered from the 2-ml marrow aspirate by centrifugation. Measurements made on the same patients are connected by a raphy. Both media were supplemented with known concentra solid line.

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Downloaded from cancerres.aacrjournals.org on October 1, 2021. © 1984 American Association for Cancer Research. Plasma Hyp and Antimetabolite Activity WI-L2 HL-60 DISCUSSION DON DON Little information is available on the concentration of Hyp in various human body fluids under physiological conditions, and the accuracy of concentrations reported previously has now been called into question by the finding that special handling of specimens is required to avoid leakage of Hyp from RBC and platelets (43). The release of Hyp during clotting probably ac counts for the fact that the mean plasma Hyp values reported here are 10-fold less than are those reported for human serum by other investigators (20, 33, 34). Although there was a wide variation in plasma Hyp concentra tions measured at random times in individual subjects, a com parison of mean values in normal subjects, solid tumor patients, and acute leukemics revealed no significant differences among these groups. In addition, in individual subjects, plasma Hyp concentration varied by an average of only 1.9-fold during a 24- hr period. These observations suggest that plasma Hyp may be regulated by metabolic controls which must affect either the entrance of Hyp into the plasma or its clearance or both. Our measurements provide some information on how changes in the production or clearance of Hyp influence plasma Hyp concentra tion. The finding of similar mean plasma Hyp concentrations in leukemic and normal subjects suggests that the increased pro duction in leukemia (30) does not exceed the clearance capacity of xanthine oxidase or urinary excretion (8, 9, 14, 30). Under normal circumstances, the clearance of Hyp via xanthine oxidase is less significant than is that by other mechanisms, since inhi bition of xanthine oxidase by HPP caused only a 1.5-fold increase in Hyp concentration. However, when HPP was combined with increased Hyp production resulting from initiation of chemother IO"6 IO"5 IO"4 IO"6 IO"5 IO"4 apy in patients with acute leukemia, there was a 7-fold increase HYPOXANTHINE CONCENTRATION in plasma Hyp concentration. Thus, under conditions of in mol/liter creased catabolic flux of purines, the activity of xanthine oxidase Chart 6. Effects of Hyp on the degree of growth inhibition produced by purine antimetaboliteson WI-L2 and HL-60cells during a 3-day culture. The antirnetabolite does have a very significant effect on plasma Hyp, and this was concentrations were irr" (A), 4 x 10"»(•),andKT5 (x) M. reflected by the good correlation between plasma Hyp and oxyourinol, which is the active metabolite of HPP present in Table 1 highest concentration in plasma after administration of HPP (14, Hyp concentration in cultures of WI-L2and HL-60 cells 16). (pufConcentration0-M)1.0 Finalconcentration The finding of a mean 4.0-fold higher concentration of Hyp in plus plus bone marrow than in venous plasma is of interest for several WI-L2ceUs"<0.1 HL-60cells'<0.1 alone1.0 reasons. First, it suggests that, in contrast to liver, which clears large amounts of Hyp from plasma on a single pass (32), bone 10 10.5 <0.1 1.1 100Medium 98Medium 18Medium 69 marrow is a net producer of Hyp. A number of investigators have 8 After 3 days of culture. concluded that bone marrow cells and gastrointestinal mucosa 0 Finalcell density was 1.5 x KP/ml. cells have a very limited capacity to carry out de novo purine c Final cell density was 0.9 x to'/mi. synthesis, and they depend on preformed purine in plasma for synthesis of purine nucleotides via the salvage pathway (1, 24, 28, 32, 39, 40). However, Hyp itself is an important regulator of are based on the initial concentration of Hyp in the culture. Table purine synthesis (17-19, 36, 38), and concentrations as low as 1 indicates that, while the concentration of Hyp remained stable 1 /im are sufficient to decrease de novo synthesis in fibroblasts in medium to which no cells were added, both WI-L2 and HL-60 (38) and marrow (22). All of the previous studies on de novo cells consumed much of the Hyp present during the 3-day period purine synthesis in marrow cells were performed under condi of growth. Thus, the average concentration of Hyp was signifi tions in which the concentration of Hyp in the cell environment cantly lower than was the initial concentration, and this was was probably quite high (1,24,28,36,39,40). Thus, rather than more marked in cultures with the lower starting Hyp concentra reflecting an inability of marrow cells to synthesize purine de tions. The impact of considering average rather than initial Hyp novo, the limited incorporation of [14C]formate used to measure concentrations on the curves depicted in Chart 6 would be to the activity of this pathway may be due to the inhibitory action flatten their slopes somewhat but, on the other hand, it indicates of the high local concentration of Hyp. If the rapid cell proliferation that average Hyp concentrations even lower than 10 fiM were in the gut is also accompanied by high local concentrations of capable of affecting purine antirnetabolite activity. Hyp, then the hypothesis that gut cells cannot produce purines

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Downloaded from cancerres.aacrjournals.org on October 1, 2021. © 1984 American Association for Cancer Research. W.E. WungandS. B. Howell via thédenovo route may also be incorrect (28). 11. Elton, G. B., Callahan, S. W., Hitchings, G. H., Rundtes, R. W., Laszlo, J. Experimental, clinical and metabolic studies of thiopurines. Cancer Chemother. Several investigators have reported previously that Hyp can Rep., 76: 197-202.1962. reverse the cytotoxicity of MMPR (2, 37). Our finding that Hyp 12. Elion, G. B., Callahan, S. W., Nathan, H., Bieber, S., Rundtes, R. W., and reduces the activity of 6-MP, 6-TG, MMPR, and DON indicates Hitching, G. H. Potentiation by inhibition of drug degradation: 6-substituted purines and xanthine oxidase. Biochem. Pharrnacol., 72: 85-93,1963. that the effect is not merely due to competition for HGPRT or 13. Elion, G. B., Callahan, S. W., Rundles, R. W., and Hitchings, G. H. Relationship for transport into the cell, since MMPR is phosphorylated by between metabolic fates and antitumor activities of thiopurines. Cancer Res., 1207-1217,1963. adenosine kinase rather than HGPRT (5, 35) and since DON 14. Elton, G. B., Kovensky, Z., Hitchings, G. H., Metz, E., and Rundles. R. W. does not require HGPRT for activation. The results of this study Metabolism studies of allopurinol, an inhibitor of xanthine oxidase. Biochem. show that the concentrations of plasma Hyp in patients receiving Pharmacol., 75: 863-880,1966. 15. Gee, T. S., Yu, K. P., and Clarkson, B. D. Treatment of adult leukemia with HPP and chemotherapy are elevated into a range which is just arabinosyl cytoxine and thtoguanine. Cancer (Phila.), 23: 1019-1032,1969. below that which begins to interfere with the cytotoxic activity 16. Hande, K., Reed, E., and Chabner, B. Allopurinol kinetics. Clin. Pharmacol. of clinically relevant concentrations of purine antimetabolites Ther., 23:598-605. 1978. 17. Henderson, F. J. Feedback inhibition of purine biosynthesis in ascites tumor against human leukemiacells in vitro. It is important to emphasize cells by purine analogues. Biochem. Pharmacol., 72: 551-556,1963. that the curves in Chart 6 were constructed using the concentra 18. Hershfield, M. S., and Seegmiller, J. E. Regulation of tie novo purine biosyn tions of Hyp present at the beginning of culture. Hyp concentra thesis in human lymphoWasts. J. Biol. Chem., 25?. 7384-7354, 1976. 19. Hershfield, M. S., and Seegmiler, J. E. Regulation of de novo purine synthesis tion decreased rapidly as cells grew and, at the end of the 3-day in human lymphoWasts. J. Biol. Chem., 252: 6002-6010,1977. culture period, there was practically no Hyp left in the medium 20. Joiley. R. L., and Scott, C. D. Preliminary results from high-resolution analysis of ultraviolet-absorbing and carbohydrate constituents in several pathological originally containing 10 MMHyp, and there was a 30 to 60% body fluids. Clin. Chem., 76: 687-696,1970. reduction of Hyp in the cultures originally containing a 100 J/M 21. Khym, J. X. Analytical system for rapid separation of tissue nucleotides at low concentration. Thus, no matter what the mechanism of the Hyp pressure on conventional anton exchanger. Clin. Chem., 27:1245-1252,1975. 22. King, M. E., Honeysett, J. M., and Howell, S. B. Regulation of de novo purine effect, the degrees of modulation depicted in Chart 6 were synthesis in human bone marrow mononuclear cells by hypoxanthine. J. Clin. actually achieved with average Hyp concentrations that were Invest., 72: 965-970,1983. considerably below the initial concentrations indicated on the 23. Kramer, W. G., and Feldman, S. High-performance liquid Chromatographie assay for allopurinol and oxipurinol in human plasma. J. Chromatogr., 762: abscissa. Although the growth conditions of leukemia cells In 92-97,1979. vitro are quite different than those in vivo, this raises the possi 24. Lajtha, L. G., and Vane, J. R. Dependence of bone marrow on the liver for purine supply. Nature (Lond.), 182: 191-192,1958. bility that the 7-fold increase in plasma Hyp that accompanied 25. LePage, G. A., and Whitecar, J. P., Jr. Pharmacology of 6-thioguanine in man. initiation of chemotherapy in acute leukemic patients receiving Cancer Res., 37:1627-1631,1971. HPP may be sufficient to reduce the effectiveness of the anti- 26. Ungston, R. B., Venditi, J. M.. Cooney, D. A., and Carter, S. K. Glutamine antagonist in chemotherapy. Adv. Pharmacol. Chemother., 8: 57-120,1970. purine antimetabolites during induction therapy. The increase in 27. Loo, T. L., Luce, J. K., Sullivan, M. P., and Frei, E., III. Clinical pharmacological Hyp also provides a potential explanation of the observation in observation on 6-mercaptopurine and 6-methylmercaptopurine riboside. Clin. rabbits that, although HPP can block the catabolism of 6-MP Pharmacol. Ther., 794: 180-194, 1967. 28. Mackinnon, A. M., and Deller, D. T. Purine nucleotide biosynthesis in gastroin (11-13) and increase the area under the plasma concentration testinal mucosa. Biochim. Biophys. Acta, 379:1-4, 1973. times time curve (41), it does not increase the toxicity of 6-MP 29. Martin, W. E., Crichton, T. K., Yag, R. C., and Evans, A. E. The metabolism of thioinosmic acid by 6-mercaptopurine sensitive and resistant leukemic leuko (42). Additional studies are required to evaluate completely the cytes. Proc. Soc. Exp. Btol. Med., 740: 423-428, 1972. role of Hyp and HPP in modulating the activity 6-MP and the 30. Merrill, D., and Jackson, H. The renal complication of leukemia. N. Engl. J. Med., 228: 271 -276, 1943. other antipurine antimetabolites in humans (6) and to validate 31. Moore, E. C., and LePage, G. A. The metabolism of 6-thtoguanine in normal the extrapolation that we have made between in vitro results and neoplastic tissues. Cancer Res., 78: 1075-1083,1958. and in vivo observations. 32. Pritchard, J. 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Downloaded from cancerres.aacrjournals.org on October 1, 2021. © 1984 American Association for Cancer Research. Hypoxanthine Concentrations in Normal Subjects and Patients with Solid Tumors and Leukemia

Wally E. Wung and Stephen B. Howell

Cancer Res 1984;44:3144-3148.

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