Clofarabine Dosing in a Patient With Acute Myeloid Leukemia on Intermittent Hemodialysis: Case Report and Review of the Literature.

Clofarabine containing chemotherapeutic regimens have demonstrated efficacy in the treatment of relapsed refractory acute myeloid leukemia. Nonetheless, there are limited data on the use of clofarabine in patients with renal failure. The present report describes the use of clofarabine in a patient with renal failure undergoing intermittent dialysis. We describe our rationale for dosing, clofarabine plasma levels obtained, and discuss our findings in the context of other available literature. Consistent with previous findings, intermittent hemodialysis was not found to be a reliable method of removing clofarabine in patients with renal insufficiency.

Simultaneous quantification of busulfan, clofarabine and F-ARA-A using isotope labelled standards and standard addition in plasma by LC-MS/MS for exposure monitoring in hematopoietic cell transplantation conditioning.

In allogeneic hematopoietic cell transplantation (HCT) it has been shown that over- or underexposure to conditioning agents have an impact on patient outcomes. Conditioning regimens combining busulfan (Bu) and fludarabine (Flu) with or without clofarabine (Clo) are gaining interest worldwide in HCT. To evaluate and possibly adjust full conditioning exposure a simultaneous analysis of Bu, F-ARA-A (active metabolite of Flu) and Clo in one analytical run would be of great interest. However, this is a chromatographical challenge due to the large structural differences of Bu compared to F-ARA-A and Clo. Furthermore, for the bioanalysis of drugs it is common to use stable isotope labelled standards (SILS). However, when SILS are unavailable (in case of Clo and F-ARA-A) or very expensive, standard addition may serve as an alternative to correct for recovery and matrix effects. This study describes a fast analytical method for the simultaneous analysing of Bu, Clo and F-ARA-A with liquid chromatography-tandem mass spectrometry (LC-MS/MS) including standard addition methodology using 604 spiked samples. First, the analytical method was validated in accordance with European Medicines Agency guidelines. The lower limits of quantification (LLOQ) were for Bu 10µg/L and for Clo and F-ARA-A 1µg/L, respectively. Variation coefficients of LLOQ were within 20% and for low medium and high controls were all within 15%. Comparison of Bu, Clo and F-ARA-A standard addition results correspond with those obtained with calibration standards in calf serum. In addition for Bu, results obtained by this study were compared with historical data analysed within TDM. In conclusion, an efficient method for the simultaneous quantification of Bu, Clo and F-ARA-A in plasma was developed. In addition, a robust and cost-effective method to correct for matrix interference by standard addition was established.

Dosing-time contributes to chronotoxicity of clofarabine in mice via means other than pharmacokinetics.

To evaluate the time- and dose-dependent toxicity of clofarabine in mice and to further define the chronotherapy strategy of it in leukemia, we compared the mortality rates, LD50s, biochemical parameters, histological changes and organ indexes of mice treated with clofarabine at various doses and time points. Plasma clofarabine levels and pharmacokinetic parameters were monitored continuously for up to 8 hours after the single intravenous administration of 20 mg/kg at 12:00 noon and 12:00 midnight by high performance liquid chromatography (HPLC)-UV method. Clofarabine toxicity in all groups fluctuated in accordance with circadian rhythms in vivo. The toxicity of clofarabine in mice in the rest phase was more severe than the active one, indicated by more severe liver damage, immunodepression, higher mortality rate, and lower LD50. No significant pharmacokinetic parameter changes were observed between the night and daytime treatment groups. These findings suggest the dosing-time dependent toxicity of clofarabine synchronizes with the circadian rhythm of mice, which might provide new therapeutic strategies in further clinical application.

Simultaneous determination of clofarabine and cytarabine in human plasma by LC-MS/MS.

Combination of cytostatic agents is a basic principle in the treatment of cancer. For the treatment of acute myeloid leukemia (AML), purine analogs, like clofarabine and cytarabine act synergistically. Little is known, however, on their interaction in vivo. We developed a method for the simultaneous determination of clofarabine and cytarabine in human plasma. The substances were extracted from plasma samples by protein precipitation with acetonitrile. Cladribine was the internal standard (IS). The analytes were separated on Synergi HydroRP column (150mm×2.0mm, 4µm) and a triple-quadrupole mass spectrometry with an electrospray ionisation (ESI) source was applied for detection. The mobile phase consisted of acetonitrile, ammonium acetate 2mM and 0.5% formic acid in a gradient mode at a flow rate of 0.5ml/min. The injection volume was 10µl and the total run time was 6.0min. Retention times were 2.46min for clofarabine, 0.97min for cytarabine and 2.43min for the IS. Calibration ranges were 8-1000ng/ml for clofarabine and 20-2500ng/ml for cytarabine. The intra-day and inter-day precision was less than 15% and the relative standard deviation was all within ±15%. This new method allows a rapid and simple determination of both clofarabine and cytarabine in human plasma. It was applied to a pharmacokinetic investigation within a hematological trial in adult patients with AML.

Preclinical examination of clofarabine in pediatric ependymoma: intratumoral concentrations insufficient to warrant further study.

Clofarabine, a deoxyadenosine analog, was an active anticancer drug in our in vitro high-throughput screening against mouse ependymoma neurospheres. To characterize the clofarabine disposition in mice for further preclinical efficacy studies, we evaluated the plasma and central nervous system disposition in a mouse model of ependymoma. A plasma pharmacokinetic study of clofarabine (45 mg/kg, IP) was performed in CD1 nude mice bearing ependymoma to obtain initial plasma pharmacokinetic parameters. These estimates were used to derive D-optimal plasma sampling time points for cerebral microdialysis studies. A simulation of clofarabine pharmacokinetics in mice and pediatric patients suggested that a dosage of 30 mg/kg IP in mice would give exposures comparable to that in children at a dosage of 148 mg/m(2). Cerebral microdialysis was performed to study the tumor extracellular fluid (ECF) disposition of clofarabine (30 mg/kg, IP) in the ependymoma cortical allografts. Plasma and tumor ECF concentration-time data were analyzed using a nonlinear mixed effects modeling approach. The median unbound fraction of clofarabine in mouse plasma was 0.79. The unbound tumor to plasma partition coefficient (K pt,uu: ratio of tumor to plasma AUCu,0-inf) of clofarabine was 0.12 ± 0.05. The model-predicted mean tumor ECF clofarabine concentrations were below the in vitro 1-h IC50 (407 ng/mL) for ependymoma neurospheres. Thus, our results show the clofarabine exposure reached in the tumor ECF was below that associated with an antitumor effect in our in vitro washout study. Therefore, clofarabine was de-prioritized as an agent to treat ependymoma, and further preclinical studies were not pursued.

Simultaneous determination of fludarabine and clofarabine in human plasma by LC-MS/MS.

A method for quantification of fludarabine (FDB) and clofarabine (CFB) in human plasma was developed with an API5000 LC-MS/MS system. FDB and CFB were extracted from EDTA plasma samples by protein precipitation with trichloroacetic acid. Briefly, 50 µL plasma sample was mixed with 25 µL internal standard (50 ng/mL aqueous 2-Cl-adensosine) and 25 µL 20% trichloroacetic acid, centrifuged at 25,000 × g (20,000 rpm) for 3 min, and then transfered to an autosampler vial. The extracted sample was injected onto an Eclipse extend C18 column (2.1 mm×150 mm, 5 µm) and eluted with 1mM NH4OH (pH 9.6) - acetonitrile in a gradient mode. Electrospray ionization in positive mode (ESI(+)) and multiple reaction monitoring (MRM) were used, and ion pairs 286/134 for FDB, 304/170 for CFB and 302/134 for the internal standard were selected for quantification. The retention times were typically 3.72 min for FDB, 4.34 min for the internal standard, 4.79 min for CFB. Total run time was 10 min per sample. Calibration range was 0.5-80 ng/mL for CFB and 2-800 ng/mL for FDB. The method was applied to a clinical pharmacokinetic study in pediatric patients.

Pharmacokinetics of clofarabine in patients with high-risk inherited metabolic disorders undergoing brain-sparing hematopoietic cell transplantation.

Clofarabine, a newer purine analog with reduced central nervous system toxicity, may prove advantageous in hematopoietic cell transplantation in patients for whom neurotoxicity is a natural part of disease progression. This study evaluated clofarabine pharmacokinetics in adult and pediatric patients undergoing hematopoietic cell transplantation for the treatment of high-risk, inherited metabolic disorders. Clofarabine (40 mg/m(2)/d) was administered intravenously on days -7 to -3. Kinetic sampling occurred with doses 1 and 5, along with a single level collected on day of transplant (day(0)). Sixteen patients were studied with a median (range) age and body surface area (BSA) of 7.5 years (0.5-43) and 0.94 m(2) (0.31-2.3), respectively. Clofarabine area under the concentration-time curve from time 0 to infinity was 931 ng·h/mL (685-1876), maximum concentration was 226 ng/mL (162-600), and minimum concentration was 3.2 ng/mL (1.7-5.6). Clofarabine clearance was 1.6 L/h/kg (0.7-2.4) and weakly correlated with weight (r(2) = 0.33) and BSA (r(2) = 0.26). No difference in plasma concentrations was found between dose 1 and dose 5 (all P > .05). All concentrations were below the limit of quantification (1 ng/mL) on day(0) in patients with normal renal function. Variability in clofarabine clearance was approximately 3-fold and was not adequately explained by covariates describing renal function and body size. In patients with adequate renal function, no drug accumulation occurs with consecutive daily dosing.

Effect of hemodialysis on the plasma levels of clofarabine.

Clofarabine, a nucleoside analogue for treatment of relapsed leukemia, is 50-60% excreted in urine. Clofarabine has not been studied in patients on hemodialysis. We measured levels in one patient in acute renal failure. Prior to dialysis, 43 hr after a 40 mg/m(2) infusion, plasma concentration was 139 ng/ml. One hour after beginning hemodialysis, a 20 mg/m(2) infusion began. Plasma concentrations were 84.2, 81.1, and 88.0 ng/ml while the dialysis and clofarabine infusion occurred simultaneously. Post-dialysis, while the clofarabine was still infusing, the level was 95.8 ng/ml. Hemodialysis does decrease clofarabine levels, but given its large volume distribution, hemodialysis may not be effective for clofarabine overdose.

Comparison of fast liquid chromatography/tandem mass spectrometric methods for simultaneous determination of cladribine and clofarabine in mouse plasma.

Several fast high performance liquid chromatography/atmospheric pressure ionization/tandem mass spectrometric (HPLC-API/MS/MS) methods were evaluated for the simultaneous determination of cladribine and clofarabine in mouse plasma samples. The chemical separation for analytes under reversed-phase conditions were achieved by using either ultra-performance liquid chromatography (UPLC) or micro-column HPLC coupled to either a quadrupole linear ion trap mass spectrometer (QTrap MS) or a triple quadrupole mass spectrometer. Atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI) interfaces in the positive mode were employed prior to mass spectrometric detection. The effects of various dopant solvents on the APPI sensitivities of analytes and the internal standard were investigated. The matrix ionization suppression potential for the test compounds in plasma samples on fast HPLC-MS/MS methods was examined by a post-column infusion technique. In this work, these proposed approaches were successfully employed to determine the concentrations of cladribine and clofarabine in mouse plasma in the low ng/ml region. The mouse plasma levels of all analytes obtained by these fast HPLC-MS/MS methods were compared and found to be well correlated in terms of analytical accuracy.

Plasma and cerebrospinal fluid pharmacokinetics of nelarabine in nonhuman primates.

INTRODUCTION: Nelarabine is a water-soluble prodrug of the cytotoxic deoxyguanosine analog ara-G, to which it is rapidly converted in vivo by adenosine deaminase. Nelarabine has shown activity in the treatment of T-cell malignancies, especially T-cell acute lymphoblastic leukemia. Preliminary data suggested that nelarabine might penetrate into the CSF. We therefore studied the CSF penetration of nelarabine and ara-G in a nonhuman primate model that has been highly predictive of anticancer drug distribution in humans. METHODS: Nelarabine (35 mg/kg, approximately 700 mg/m2) was administered over 1 h through a surgically implanted central venous catheter to four nonhuman primates. Blood (four animals) and ventricular CSF (three animals) samples were obtained at intervals for 24 h for determination of nelarabine concentrations, which were measured by HPLC-mass spectrometry. RESULTS: The nelarabine plasma AUC (median+/-s.d.) was 2,820+/-1,140 microM min and the ara-G plasma AUC was 20,000+/-8,100 microM min. The terminal half-life of nelarabine in plasma was 25+/-5.2 min and clearance was 42+/-61 ml/min/kg. The terminal half-life of ara-G in plasma was 182+/-45 min. In CSF the terminal half-life of nelarabine was 77+/-28 min and of ara-G was 232+/-79 min. The AUCcsf:AUCplasma was 29+/-11% for nelarabine and 23+/-12% for ara-G. CONCLUSION: The excellent CSF penetration of nelarabine and ara-G supports further study of the contribution of nelarabine to the prevention and treatment of CNS leukemia.

Safety assessment of 4'-thio-beta-D-arabinofuranosylcytosine in the beagle dog suggests a drug-induced centrally mediated effect on the hypothalamic-pituitary-adrenal axis.

4'-Thio-beta-D-arabinofuranosylcytosine (OSI-7836) is a nucleoside analogue with structural similarity to gemcitabine and cytarabine (ara-C). Myelosuppression, reversible transaminase elevations, and flu-like symptoms are common side effects associated with human use of gemcitabine and ara-C. Fatigue is also associated with the use of gemcitabine and OSI-7836 in humans. To better understand the toxicity of OSI-7836, subchronic studies were conducted in dogs. OSI-7836 was administered on days 1 and 8 or on days 1, 2, and 3 of a 21-day dose regimen. These schedules attempted to match clinical trial dosing regimens. Routine toxicity study end points demonstrated that OSI-7836 was primarily cytotoxic to the gastrointestinal tract, bone marrow, and testes; the myelotoxicity was mild and reversible. Plasma pharmacokinetics were dose-linear with an elimination half-life of 2.2 h. Follow-up single dose experiments in dogs assessed drug effects on lymphocyte subpopulations and on adrenal and thyroid function. Populations of T and B cells were equally reduced following OSI-7836 administration. There were no adverse effects on thyroid function, but there were marked reductions in circulating cortisol and adrenocorticotropic hormone concentrations suggesting a centrally mediated impairment of the hypothalamic-pituitary-adrenal axis. These findings show a toxicological profile with OSI-7836 similar to other nucleoside analogues and suggest that the beagle is a model for studying one possible cause of OSI-7836-related fatigue, impaired function of the hypothalamic-pituitary-adrenal axis.

Plasma and cerebrospinal fluid pharmacokinetics of clofarabine in nonhuman primates.

INTRODUCTION: Clofarabine (2-chloro-2'fluoro-2'-deoxy-9-beta-d-arabinofuranosyladenine) is a purine nucleoside analogue that is active in the treatment of acute leukemia. We studied the pharmacokinetics and cerebrospinal fluid penetration of clofarabine in a nonhuman primate model. METHODS: A dose of 2.3 mg/kg of clofarabine was given i.v. over 2 hours to each of four animals. Plasma and cerebrospinal fluid (CSF) samples were obtained at specified intervals and the clofarabine concentration determined by reverse-phase high-pressure liquid chromatography with mass spectroscopy. RESULTS: The median clofarabine clearance was 17 mL/min/kg (range, 15-20), the median plasma area under the concentration-time curve was 452 mumol/L minutes (range, 380-487), and the median terminal half-life was 105 minutes (range, 78-138). Concentrations of clofarabine in CSF could not be modeled reliably because the terminal rate constant was not well defined. The median CSF penetration was 5% (range, 3-26%). CONCLUSION: Clofarabine penetrates into the CSF only modestly, but the concentrations obtained may approach those that are cytotoxic in vitro. Evaluation of the contribution of clofarabine to central nervous system preventive therapy should be considered in future studies.

Stability and analysis of 2-chloro-2'-deoxyadenosine, 2-chloro-2'-arabino-fluoro-2'-deoxyadenosine and 2-chloroadenine in human blood plasma.

Cladribine (2-chloro-2'-deoxyadenosine, CdA) is a purine nucleoside analog with activity against lymphoproliferative and autoimmune disorders. 2-Chloro-2'-arabino-fluoro-2'-deoxyadenosine (CAFdA), a derivative of CdA with better acid stability, shows a similar in vitro spectrum of activity as CdA. 2-Chloroadenine (CAde) is the major catabolite of both CdA and CAFdA. We have developed a high performance liquid chromatography method to measure CdA, CAFdA and their metabolite CAde in plasma. This method employees an internal standard, chloroadenosine (CAdo), and a C8 solid-phase extraction to isolate and concentrate the substances. Chromatographic separation was achieved using a C8 reverse-phase column, with UV detection at 265 nm, which gives a limit of detection of 1 nmol/l for all substances. The method was reproducible with intra- and inter-assay coefficients of variations below 6% at 50 nmol/l and at 5 nmol/l below 23%. The average recoveries of CdA, CAde, CAFdA and the internal standard were higher than 70%. Stability studies of authentic patient samples show that samples containing CdA should be kept in a refrigerator or on ice to prevent degradation. Plasma containing CAde should not be kept at -20 degrees C for longer than 10 weeks before analysis. CdA and CAFdA remain almost stable during storage at -20 degrees C for 12 weeks.

Pharmacological and biochemical strategies to increase the accumulation of arabinofuranosylguanine triphosphatein primary human leukemia cells.

Purine nucleoside phosphorylase deficiency leads to a dGTP-mediated T-lymphopenia, suggesting that an analogue of deoxyguanosine would be selectively effective in T-cell disease. 9-beta-D-Arabinofuranosylguanine (ara-G) is relatively resistant to hydrolysis by purine nucleoside phosphorylase and selectively toxic to T cells, but its low solubility has prevented its use in the clinic. 2-Amino-6-methoxy-arabinofuranosylpurine (GW506U) serves as the water-soluble prodrug for ara-G. A Phase I trial in patients with refractory hematological malignancies demonstrated that the clinical responses to this agent were directly related to the peak levels of ara-G 5'-triphosphate (ara-GTP) in target cells. The aim of the present study was to develop and test strategies to increase intracellular accumulation of ara-GTP in primary human leukemia cells of myeloid and B-lymphoid origin. Three strategies were tested. First, incubations with 100 microM ara-G for 4 h produced a linear median accumulation rate of 19 microM/h (range, 2-45 microM/h; n = 15) in lymphoid leukemia cells and 16 microM/h (range, 0.5-41 microM/h; n = 11) in myeloid leukemia cells. Saturation of ara-GTP accumulation was achieved only after 6-8 h exposure in both lymphoid and myeloid leukemia cells, suggesting a rationale for prolonged infusion. Second, a dose-dependent increase in ara-GTP accumulation was observed with incubations of 10-300 microM ara-G for 3 h. Hence, dosing regimens that achieve high plasma levels of ara-G during therapy may increase cellular levels of ara-GTP. Finally, a biochemical modulation approach using in vitro incubation of leukemia cells with 10 microM 9-beta-D-arabinofuranosyl-2-fluoroadenine for 3 h, followed by either 50 or 100 microM ara-G for 4 h, resulted in a statistically significant median 1.3-fold (range, 1.1-9.0-fold; P = 0.034) and 1. 8-fold (range, 0.9-10.6 fold; P = 0.018) increase in ara-GTP compared to cells incubated with ara-G alone. Extension of these studies to ex vivo incubations confirmed our in vitro findings. These strategies will be used in the design of clinical protocols to increase ara-GTP accumulation in leukemia cells during therapy.

Kinetics and metabolism of 2-chloro-2'-deoxyadenosine and 2-chloro-2'-arabino-fluoro-2'-deoxyadenosine in the isolated perfused rat liver.

2-Chloro-2'-deoxyadenosine (CdA), a newly developed anticancer drug, has been tested in phase II trials in the treatment of lymphoproliferative disorders. 2-Chloro-2'-arabino-fluoro-2'-deoxyadenosine (CAFdA), an acid stable derivative of CdA with promising anti-lymphoproliferative activity, has been suggested as a potentially effective oral drug. In the present study, we investigated the metabolism of CdA and CAFdA in isolated perfused rat liver. The liver was recycled with a perfusate containing CdA or CAFdA (2-200 micrograms/ml) for 3.5 h. The elimination half-lives were concentration-dependent for both CdA and CAFdA. The elimination rate of CAFdA was slower than that of CdA, suggesting that CAFdA is more stable than CdA against deglycosylation by hepatic enzymes. The amount of 2-chloroadenine (CAde), the major metabolite of CdA and CAFdA, increased proportionally with time and dose. The first passage effect was approximately 50% both for CdA and CAFdA. Less than 1% of CdA and CAFdA were recovered as intact drug in the bile during the experiment and less than 1% of CdA and 0.1% of CAFdA were found as CAde in the bile, respectively. The structural identity of metabolites present in the perfusates was verified utilizing electrospray ionization mass spectrometry.

Transport and metabolism of 9-beta-D-arabinofuranosylguanine in a human T-lymphoblastoid cell line: nitrobenzylthioinosine-sensitive and -insensitive influx.

Nitrobenzylthioinosine (NBMPR), dipyridamole, and dilazep, potent inhibitors of nucleoside transport, were found to be ineffective in preventing 9-beta-D-arabinofuranosylguanine (ara-G)-induced inhibition of MOLT 4 and CCRF CEM cell growth, ara-G (2.0 microM) was metabolized to 9-beta-D-arabinofuranosylguanine 5'-triphosphate in MOLT 4 cells, and the levels of this metabolite were not affected by the presence of 5.0 microM NBMPR in the incubation medium. Permeation of the MOLT 4 cell membrane by ara-G occurred primarily by means of the NBMPR-sensitive nucleoside transport system. However, a residual transport component accounting for 10-20% of the total transport activity was demonstrated in the presence of NBMPR. This component was inhibited by adenine and hypoxanthine but not by dilazep, dipyridamole, or other nucleosides. In contrast, inhibitors of nucleoside transport readily reversed the cytotoxic effect of 7-deazaadenosine (tubercidin) in both MOLT 4 and CCRF CEM cells. The levels of tubercidin 5'-triphosphate formed from 2.0 microM tubercidin in MOLT 4 cells were reduced by 80% in the presence of 5.0 microM NBMPR. The influx of tubercidin into MOLT 4 cells was found to occur primarily by means of the NBMPR-sensitive nucleoside transport system. This same system mediated the transport of ara-G into human erythrocytes.

Renal handling and production of plasma and urinary adenosine.

The present study was undertaken to determine the renal handling of plasma adenosine and the relative contribution of the kidney to the adenosine in the renal venous plasma and urine. Injections of radiolabeled adenosine, as a tracer of arterial adenosine, along with reference compounds (either inulin or 9-beta-D-arabinofuranosyl hypoxanthine, an analogue of adenosine that does not occupy the nucleoside carrier) were coupled with measurements of endogenous adenosine in the arterial and renal venous plasma and urine of 11 anesthetized dogs. The arterial and venous concentration of endogenous adenosine was 60 +/- 16 and 52 +/- 10 nM, respectively. Urinary adenosine concentration was 312 +/- 53 nM and the fractional excretion was 0.71 +/- 0.14. Of the radiolabeled adenosine injected into the renal artery, approximately 53 +/- 3% of the filtered tracer was recovered in the urine, and only 11 +/- 1% of the tracer was recovered in the venous plasma. These results demonstrate uptake of adenosine from both the tubular and vascular compartments, and analysis of single-injection multiple-indicator curves indicates that a substantial amount of the extracted arterial adenosine enters and remains in cells. We conclude that arterial plasma contributes significantly to adenosine excreted in the urine but only minimally to renal venous adenosine. Furthermore, any intervention that alters cellular uptake and metabolism of adenosine may lead to significant changes in extracellular adenosine.

Effect of liver disease on pharmacokinetics and toxicity of 9-beta-D-arabinofuranosyladenine-5-phosphate.

Plasma levels of 9-beta-D-arabinofuranosyladenine-5'-phosphate (ara-AMP) and its metabolites 9-beta-D-arabinofuranosyladenine (ara-A and 9-beta-D-arabinofuranosylhypoxanthine (ara-Hx) were determined by high-performance liquid chromatography in four patients with chronic active hepatitis positive for hepatitis B surface antigen and eight patients with severe herpesvirus infections and normal liver function. Ara-AMP was given intravenously over a 30-min period in doses ranging from 10 to 30 mg of ara-A equivalent/kg per day. The metabolism of ara-AMP did not differ significantly between the two patient groups. Ara-AMP was quickly converted to ara-A, which was rapidly deaminated to ara-Hx. The mean half-lives of ara-AMP, ara-A, and ara-Hx were 0.14 hr, 0.17 hr, and 3.5 hr, respectively. Thus, ara-AMP is rapidly metabolized and does not act as a depot form of ara-A. Patients with chronic active hepatitis demonstrated increased bone marrow sensitivity to ara-AMP and a musculoskeletal pain syndrome not observed in patients treated for herpesvirus infections.