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Renal function after acyclovir intravenous injection
Renal Function after Acyclovir Intravenous Injection
DAVID BRIGDEN, M.B., M.R.C.P. ANN E. ROSLING, B.Sc. NICHOLAS C. WOODS, M.I.Biol. Beckenham,
England
Plasma urea or creatinine was noted to be raised in 58 of 354 patients treated with intravenous acyclovir. This occurred after intravenous bolus injection of high dosages of acyclovir but the risk was considerably reduced by slow intravenous infusion of the same dosage over one hour, with adequate hydration of the patient and adjustment of dosage in patients with preexisting impaired renal function. Animal studies indicate that the impairment of renal function associated with high bolus injections of acyclovir is due to crystal formation in the renal tubules and/or the collecting ducts, and that the crystals are removed after cessation of treatment. The special problems involved in the treatment of patients with herpes encephalitis necessitating limited fluid intake and the possible interaction with other nephrotoxic drugs are discussed. Animal studies with high dosages of intravenous acyclovir suggest that renal damage can occur [ 11. Twenty mg/kg doses in rats produced obstructive crystal nephropathy and similar dosages in dogs caused increased water intake and urine output with a reduced urine specific gravity and osmolarity. Selby et al. [2] first reported impaired renal function during treatment with acyclovir in humans. They described two patients, both with disseminated zoster and poor fluid intake, in whom blood urea levels rose to 11 .O and 15.3 mmol/liter, respectively. Renal function recovered when they were given a reduced dosage combined with additional fluids. METHODS
From the Wellcome Research Laboratories, Beckenham,Kent,England.Requestsfor reprints shouldbe addressedto Dr. David Brigden,Wellcome Research Laboratories, Langley Court, Beckenham, Kent, England.
182
Clinical Experience. Reports of 354 patients treated with intravenous acyclovir for potentially life-threatening infections with herpes simplex, varicella zoster, cytomegalo-, and Epstein-Barr viruses were carefully scrutinized. All records of adverse events were noted. Individual doses varied from 2.5-10 mg/kg and were usually administered at eight hourly intervals for five days, but were reduced in patients with preexisting renal impairment. However, these guidelines were not always observed. Initially the drug was administered by bolus intravenous injection although later patients in this series were treated by slow intravenous infusion over one hour. Animal Studies. During an experiment conduoted for other purposes, the oresence of crystals was determined in mouse kidneys. Eighteen male BKA mice were given 50 mg/kg acyclovir by intraperitoneal injection twice daily for five days. Animals were sacrificed at the following times after the last dose: five at 10 minutes, five at 30 minutes, four at 60 minutes, and four at 120 minutes. Frozen sections and hematoxylin and eosin stained paraffin sections were prepared.
Acyclovlr Symposium The American Journal of Medicine
ACYCLOVIR PHARMACOKINETICS AND TOLERANCE IN MAN-BRIGDEN
TABLE I
ET AL.
Renal Impairment in Patients Receiving One-Hour Infusions of Acyclovir
Sex
Age
infection*
F F M F M M M M F F M F
27 36 68 60 44 9 60 35 43 55 19 66
Enc Enc HSV Enc Enc Enc Enc WV Enc Enc HSV HSV
Preexisting Renal impairment
Severity+
Yes
-
Yes
-
Yes
+
Yes -
-
Yes -
+ -
Yes Yes Yes
+
Yes -
+
Yes Yes
?
-
-
Documented Return of Renal Functionto Pretreatment Levels*
-
-
Alternative Causesfor Changesin Renal Function
-
Yes Yes Yes -
ENC = encephalitis; HSV = herpes simplex virus; VZV = variceffa zoster virus. + -f- = blood urea >12 mmof/fiter and/or serum creatinine >240 fimof/fiter. t Yes = return to normal, no comment = outcome unknown. l
Patients. Fifty-eight patients experienced rises in plasma urea and/or creatinine levels either above the normal range (creatinine > 133 ~mol/liter; urea >6.6 mmol/liter) or significantly above already high levels. Forty-six of these patients received the drug by bolus intravenous injection and 12 by intravenous infusion over one hour (Table I). Sample case reports follow. Case I: A three year old girl was treated for herpes encephalitis with 10 mg/kg acyclovir at eight hourly intervals for nine days. Immediately before therapy her plasma urea level was 3.6 mmol/liter and plasma creatinine 50 ~mollliter; after six doses the values were 12.0 and 240, respectively. After 12 doses, however, the plasma urea had dropped to 10.0 mmol/liter and the plasma creatinine had returned to normal (48 pmol/liter). Twenty-four hours after the last dose the plasma urea had also returned to within normal limits. Case 2: A man aged 24 years was treated for herpes zoster with intravenous acyclovir 10 mg/kg at eight hourly intervals for 11 doses. After the first three doses his plasma urea had risen from 7.5 to 12.5 mmol/liter and his plasma creatinine from 143 to 205 ~mol/liter. After six doses the urea level had risen further to 25.4 mrnol/liter and the creatinine to 293 pmol/liter. Twelve hours after the final dose the urea had dropped to 9.5 mmol/liter and the creatinine to 173 PmoMiter. Two days later the values were 8.5 and 145, respectively. Case 3: A 70 year old man was treated for herpes encephalitis initially by intravenous bolus injection of 10 mg/kg every eight hours. After six doses his plasma urea had risen from 8.7 mmol/liter to 33.9 mmol/liter
and his creatinine from 93 to 572 FmoVliter. Progressive dosage reduction was then instituted to 7.5 mg/kg every 24 hours by slow intravenous infusion over one hour. After 10 days of therapy his plasma urea had returned to 9.7 mmol/liter and his creatinine to 110 ~mol/liter. Seventeen days later both his blcod urea and creatinine levels were within normal limits. Forty-six patients who received bolus injections of acyclovir experiencad blood urea and/or creatinine level increases that could have been due to drug administration. One patient, a neonate who died of an overwhelming virus infection, had no evidence of renal impairment immediately before death but birefringement crystals were found in the renal tubules at postmortem examination. Twenty-seven (59 percent) of these 46 patients showed a pattern of rapid rise in blood urea, sometimes accompanied by an increase in creatinine, within 24 to 48 hours after starting therapy, followed by a tendency to return to normal despite continuing treatment. Blood ureajcreatinine returned to normal in 22 of these 27 patients. In two of these 22 patients increases in blood ureaicreatinine occurred when drug was administered by a bolus intravenous injection and returned to normal when drug was administered by infusion (in case 3 above, with a reduction in dosage and frequency). Of the other five patients, renal function returned towards normal in two patients and returned to the preexisting abnormal blood urea level in two others, while information was not available for the remaining patient. Nine of these 27 patients experienced severe renal impairment as defined by blood urea and/or creatinine levels above 12 mrnol/liter or 240 PM/liter, respectively.
Tfw American
Journal
ol Medkine
Acycfovlr
Sympoelum
183
ACYCLOVIR
PHARMACOKINETICS
TABLE II
Effect of Acyclovir (50 mg Injected lntraperitoneally) on Renal Tubules in the Mouse
Time Alter LastDose(mins) 10 10 10 10 10 30 30 30 30 30 69 60 60 60 120 120 120 120 l
AND TOLERANCE IN MAN-BRIGDEN
lntratubular Crystals -
PlasmaAcyclovir Level(p&f)* 54 135 144 207 214 92 a4 77 71 58 14 41 38 30 14 1.4 1.4 2.1
++ f f + ++ +++ +++ +++ f ++ + ++ ++ -
Determined by radioimmunoassay [3].
Fourteen of the remaining 19 patients experienced increases in blood urea/creatinine for different reasons: administration of cyclosporin A, gentamicin, amphotericin B, or “aminoglycoside” antibiotics to 10 patients; administration of digoxin, chlorambucil, and steroids to one; streptococcal septicemia and gastrointestinal hemorrhage in one patient; and gram-negative septicemia in one patient. The changes in blood urea were within the limits of laboratory variation in a further patient. Of the other five patients, one died of encephalitis on the third day of treatment and one died of encephalitis/cardiac arrest on the fourth day. One further patient with a virus encephalitis was treated with mannitol. One patient with a cardiac transplant received three courses of acyclovir. His increase in a blood urea during the first course of treatment was barely significant; he experienced a modest increase during the second course and no increase in the final course. Follow-up information was not available for the fifth patient. Twelve patients had renal impairment while receiving acyclovir as’an infusion (Table I). Four patients had alternative causes for a decline in renal function. These and one other patient had evidence of preexisting renal impairment. Recommendations for a reduced frequency of dosage were not followed, leading to overdosages. The remaining seven patients were being treated for encephalitis and none had evidence of preexisting renal impairment. In five patients blood urea/creatinine levels returned to normal as treatment was continued. One other patient had evidence of a return toward normal,
184
Acyclovir Symposium
The American Journal of Medicine
ET AL
while the remaining patient died of a cerebral vascular accident on the third day of treatment with a blood urea of 7.8 mmol/liter and a normal level of serum creatinine. Animal Studies. Crystals were observed in the collecting tubules of the papilla predominantly at its tip. Considerable variation was seen within the subgroups but a general pattern of crystal deposition versus time emerged. Crystallization was slight at 10 minutes, but most of the collecting tubules in the papilla were packed with refractile material by 30 minutes. Crystals decreased in number by 60 minutes and by 120 minutes most of the animals showed no intratubular crystals, although some remained in the pelvis (Table II). Minimal histologic changes were seen in the stained sections consisting of basophilia, swelling, and occasional desquamation of collecting tubule epithelial cells with some interstitial edema. The changes were considered likely to be completely reversible on cessation of treatment. COMMENTS The animal studies suggest that the increases in blood urea/creatinine seen in some patients treated with acyclovir are caused by a transient crystal nephropathy, which occurs when the concentration of acylovir in the collecting duct of the tubule exceeds the drug’s solubility. This probably occurs for short periods when the acyclovir plasma concentrations are very high immediately after a botus injection. Peak plasma levels are reduced when acyclovir is given by slow intravenous infusion over one hour. Since this dosing regime was instituted fewer cases of renal impairment have been reported. The cases that have occurred have either been in patients with preexisting renal failure in whom dosage reduction advice had been ignored or in patients with herpes encephalitis. Relative dehydration is probably an important contributory factor. Patients hospitalized with severe herpes infections are frequently dehydrated which leads to a high degree of urinary concentration in the renal tubules and collecting ducts. This would lead to an increase in the likelihood of acyclovir crystal formation and a consequent failure of renal function. The patient’s dehydration is usually rectified by hospital care with a consequent decrease in crystal formation and an improvement in renal function. Patients with herpes encephalitis, however, present a special problem. Many physicians treating this disease either restrict fluid intake or even use diuretics to produce relative dehydration as part of the therapy for brain edema. It may, therefore, prove a necessary risk if patients with herpes encephalitis are to be treated with acyclovir. A further problem which may be posed by the use of acyclovir will be the concomitant use of nephrotoxic
ACYCLOVIR PHARMACOKINETICSAND TOLERANCE IN MAN-BRIGDEN ET AL.
drugs such as the aminoglycoside antibiotics, or the new immunosuppressive agent, cyclosporin A. Little is known about these potential interactions so particular care should be observed when using these drugs concurrently. In conclusion, dosage-related impairment of renal function can occur after bolus intravenous administration of acyclovir. This can be prevented by the use of slow intravenous infusion, careful attention to hydration, and dosage reduction in patients with preexisting abnormal renal function. Particular care is required when treating patients with herpes encephalitis who require
fluid restriction or diuretic therapy and also in patients on potentially nephrotoxic drugs. ACKNOWLEDGMENT Grateful thanks are extended to all the physicians who have kindly supplied the information about the patients in this report, to D. A. Bye for carrying out the acyclovir estimation, to Dr. D. S. Freestone and Dr. G. D. W. McKendrick for much help and advice, and especially to Carole Ferdinand0 for tolerance beyond the call of duty in collecting the information and preparing the manuscript.
REFERENCES 1. Tucker WE,Macklin AW, Szot RJ, Clive D: Preclinical safety
2.
evaluation of Zovirax (BW 24811). 18th Interscience Conference on Antimicrobial Agents and Chemotherapy, 1978. Abs. 64. Washington, D.C., American Society for Microbiology. Selby PJ, Powles RL, Jameson B, et al.: Parenteral acylovir
3.
therapy for herpesvirus infections in man. Lancet 1979; II: 1267-1270. Quinn BP, de Miranda P, Gerald L, Good SS: A sensitive radioimmunoassay for the antiviral agent BW248U (9-(2-hydroxyethoxymethyhguanine). Anal Biochem 1979; 98: 319-328.
The American Journal ol Medicine
Acyclovir Sympoelum
185
DAVID BRIGDEN, M.B., M.R.C.P. ANN E. ROSLING, B.Sc. NICHOLAS C. WOODS, M.I.Biol. Beckenham,
England
Plasma urea or creatinine was noted to be raised in 58 of 354 patients treated with intravenous acyclovir. This occurred after intravenous bolus injection of high dosages of acyclovir but the risk was considerably reduced by slow intravenous infusion of the same dosage over one hour, with adequate hydration of the patient and adjustment of dosage in patients with preexisting impaired renal function. Animal studies indicate that the impairment of renal function associated with high bolus injections of acyclovir is due to crystal formation in the renal tubules and/or the collecting ducts, and that the crystals are removed after cessation of treatment. The special problems involved in the treatment of patients with herpes encephalitis necessitating limited fluid intake and the possible interaction with other nephrotoxic drugs are discussed. Animal studies with high dosages of intravenous acyclovir suggest that renal damage can occur [ 11. Twenty mg/kg doses in rats produced obstructive crystal nephropathy and similar dosages in dogs caused increased water intake and urine output with a reduced urine specific gravity and osmolarity. Selby et al. [2] first reported impaired renal function during treatment with acyclovir in humans. They described two patients, both with disseminated zoster and poor fluid intake, in whom blood urea levels rose to 11 .O and 15.3 mmol/liter, respectively. Renal function recovered when they were given a reduced dosage combined with additional fluids. METHODS
From the Wellcome Research Laboratories, Beckenham,Kent,England.Requestsfor reprints shouldbe addressedto Dr. David Brigden,Wellcome Research Laboratories, Langley Court, Beckenham, Kent, England.
182
Clinical Experience. Reports of 354 patients treated with intravenous acyclovir for potentially life-threatening infections with herpes simplex, varicella zoster, cytomegalo-, and Epstein-Barr viruses were carefully scrutinized. All records of adverse events were noted. Individual doses varied from 2.5-10 mg/kg and were usually administered at eight hourly intervals for five days, but were reduced in patients with preexisting renal impairment. However, these guidelines were not always observed. Initially the drug was administered by bolus intravenous injection although later patients in this series were treated by slow intravenous infusion over one hour. Animal Studies. During an experiment conduoted for other purposes, the oresence of crystals was determined in mouse kidneys. Eighteen male BKA mice were given 50 mg/kg acyclovir by intraperitoneal injection twice daily for five days. Animals were sacrificed at the following times after the last dose: five at 10 minutes, five at 30 minutes, four at 60 minutes, and four at 120 minutes. Frozen sections and hematoxylin and eosin stained paraffin sections were prepared.
Acyclovlr Symposium The American Journal of Medicine
ACYCLOVIR PHARMACOKINETICS AND TOLERANCE IN MAN-BRIGDEN
TABLE I
ET AL.
Renal Impairment in Patients Receiving One-Hour Infusions of Acyclovir
Sex
Age
infection*
F F M F M M M M F F M F
27 36 68 60 44 9 60 35 43 55 19 66
Enc Enc HSV Enc Enc Enc Enc WV Enc Enc HSV HSV
Preexisting Renal impairment
Severity+
Yes
-
Yes
-
Yes
+
Yes -
-
Yes -
+ -
Yes Yes Yes
+
Yes -
+
Yes Yes
?
-
-
Documented Return of Renal Functionto Pretreatment Levels*
-
-
Alternative Causesfor Changesin Renal Function
-
Yes Yes Yes -
ENC = encephalitis; HSV = herpes simplex virus; VZV = variceffa zoster virus. + -f- = blood urea >12 mmof/fiter and/or serum creatinine >240 fimof/fiter. t Yes = return to normal, no comment = outcome unknown. l
Patients. Fifty-eight patients experienced rises in plasma urea and/or creatinine levels either above the normal range (creatinine > 133 ~mol/liter; urea >6.6 mmol/liter) or significantly above already high levels. Forty-six of these patients received the drug by bolus intravenous injection and 12 by intravenous infusion over one hour (Table I). Sample case reports follow. Case I: A three year old girl was treated for herpes encephalitis with 10 mg/kg acyclovir at eight hourly intervals for nine days. Immediately before therapy her plasma urea level was 3.6 mmol/liter and plasma creatinine 50 ~mollliter; after six doses the values were 12.0 and 240, respectively. After 12 doses, however, the plasma urea had dropped to 10.0 mmol/liter and the plasma creatinine had returned to normal (48 pmol/liter). Twenty-four hours after the last dose the plasma urea had also returned to within normal limits. Case 2: A man aged 24 years was treated for herpes zoster with intravenous acyclovir 10 mg/kg at eight hourly intervals for 11 doses. After the first three doses his plasma urea had risen from 7.5 to 12.5 mmol/liter and his plasma creatinine from 143 to 205 ~mol/liter. After six doses the urea level had risen further to 25.4 mrnol/liter and the creatinine to 293 pmol/liter. Twelve hours after the final dose the urea had dropped to 9.5 mmol/liter and the creatinine to 173 PmoMiter. Two days later the values were 8.5 and 145, respectively. Case 3: A 70 year old man was treated for herpes encephalitis initially by intravenous bolus injection of 10 mg/kg every eight hours. After six doses his plasma urea had risen from 8.7 mmol/liter to 33.9 mmol/liter
and his creatinine from 93 to 572 FmoVliter. Progressive dosage reduction was then instituted to 7.5 mg/kg every 24 hours by slow intravenous infusion over one hour. After 10 days of therapy his plasma urea had returned to 9.7 mmol/liter and his creatinine to 110 ~mol/liter. Seventeen days later both his blcod urea and creatinine levels were within normal limits. Forty-six patients who received bolus injections of acyclovir experiencad blood urea and/or creatinine level increases that could have been due to drug administration. One patient, a neonate who died of an overwhelming virus infection, had no evidence of renal impairment immediately before death but birefringement crystals were found in the renal tubules at postmortem examination. Twenty-seven (59 percent) of these 46 patients showed a pattern of rapid rise in blood urea, sometimes accompanied by an increase in creatinine, within 24 to 48 hours after starting therapy, followed by a tendency to return to normal despite continuing treatment. Blood ureajcreatinine returned to normal in 22 of these 27 patients. In two of these 22 patients increases in blood ureaicreatinine occurred when drug was administered by a bolus intravenous injection and returned to normal when drug was administered by infusion (in case 3 above, with a reduction in dosage and frequency). Of the other five patients, renal function returned towards normal in two patients and returned to the preexisting abnormal blood urea level in two others, while information was not available for the remaining patient. Nine of these 27 patients experienced severe renal impairment as defined by blood urea and/or creatinine levels above 12 mrnol/liter or 240 PM/liter, respectively.
Tfw American
Journal
ol Medkine
Acycfovlr
Sympoelum
183
ACYCLOVIR
PHARMACOKINETICS
TABLE II
Effect of Acyclovir (50 mg Injected lntraperitoneally) on Renal Tubules in the Mouse
Time Alter LastDose(mins) 10 10 10 10 10 30 30 30 30 30 69 60 60 60 120 120 120 120 l
AND TOLERANCE IN MAN-BRIGDEN
lntratubular Crystals -
PlasmaAcyclovir Level(p&f)* 54 135 144 207 214 92 a4 77 71 58 14 41 38 30 14 1.4 1.4 2.1
++ f f + ++ +++ +++ +++ f ++ + ++ ++ -
Determined by radioimmunoassay [3].
Fourteen of the remaining 19 patients experienced increases in blood urea/creatinine for different reasons: administration of cyclosporin A, gentamicin, amphotericin B, or “aminoglycoside” antibiotics to 10 patients; administration of digoxin, chlorambucil, and steroids to one; streptococcal septicemia and gastrointestinal hemorrhage in one patient; and gram-negative septicemia in one patient. The changes in blood urea were within the limits of laboratory variation in a further patient. Of the other five patients, one died of encephalitis on the third day of treatment and one died of encephalitis/cardiac arrest on the fourth day. One further patient with a virus encephalitis was treated with mannitol. One patient with a cardiac transplant received three courses of acyclovir. His increase in a blood urea during the first course of treatment was barely significant; he experienced a modest increase during the second course and no increase in the final course. Follow-up information was not available for the fifth patient. Twelve patients had renal impairment while receiving acyclovir as’an infusion (Table I). Four patients had alternative causes for a decline in renal function. These and one other patient had evidence of preexisting renal impairment. Recommendations for a reduced frequency of dosage were not followed, leading to overdosages. The remaining seven patients were being treated for encephalitis and none had evidence of preexisting renal impairment. In five patients blood urea/creatinine levels returned to normal as treatment was continued. One other patient had evidence of a return toward normal,
184
Acyclovir Symposium
The American Journal of Medicine
ET AL
while the remaining patient died of a cerebral vascular accident on the third day of treatment with a blood urea of 7.8 mmol/liter and a normal level of serum creatinine. Animal Studies. Crystals were observed in the collecting tubules of the papilla predominantly at its tip. Considerable variation was seen within the subgroups but a general pattern of crystal deposition versus time emerged. Crystallization was slight at 10 minutes, but most of the collecting tubules in the papilla were packed with refractile material by 30 minutes. Crystals decreased in number by 60 minutes and by 120 minutes most of the animals showed no intratubular crystals, although some remained in the pelvis (Table II). Minimal histologic changes were seen in the stained sections consisting of basophilia, swelling, and occasional desquamation of collecting tubule epithelial cells with some interstitial edema. The changes were considered likely to be completely reversible on cessation of treatment. COMMENTS The animal studies suggest that the increases in blood urea/creatinine seen in some patients treated with acyclovir are caused by a transient crystal nephropathy, which occurs when the concentration of acylovir in the collecting duct of the tubule exceeds the drug’s solubility. This probably occurs for short periods when the acyclovir plasma concentrations are very high immediately after a botus injection. Peak plasma levels are reduced when acyclovir is given by slow intravenous infusion over one hour. Since this dosing regime was instituted fewer cases of renal impairment have been reported. The cases that have occurred have either been in patients with preexisting renal failure in whom dosage reduction advice had been ignored or in patients with herpes encephalitis. Relative dehydration is probably an important contributory factor. Patients hospitalized with severe herpes infections are frequently dehydrated which leads to a high degree of urinary concentration in the renal tubules and collecting ducts. This would lead to an increase in the likelihood of acyclovir crystal formation and a consequent failure of renal function. The patient’s dehydration is usually rectified by hospital care with a consequent decrease in crystal formation and an improvement in renal function. Patients with herpes encephalitis, however, present a special problem. Many physicians treating this disease either restrict fluid intake or even use diuretics to produce relative dehydration as part of the therapy for brain edema. It may, therefore, prove a necessary risk if patients with herpes encephalitis are to be treated with acyclovir. A further problem which may be posed by the use of acyclovir will be the concomitant use of nephrotoxic
ACYCLOVIR PHARMACOKINETICSAND TOLERANCE IN MAN-BRIGDEN ET AL.
drugs such as the aminoglycoside antibiotics, or the new immunosuppressive agent, cyclosporin A. Little is known about these potential interactions so particular care should be observed when using these drugs concurrently. In conclusion, dosage-related impairment of renal function can occur after bolus intravenous administration of acyclovir. This can be prevented by the use of slow intravenous infusion, careful attention to hydration, and dosage reduction in patients with preexisting abnormal renal function. Particular care is required when treating patients with herpes encephalitis who require
fluid restriction or diuretic therapy and also in patients on potentially nephrotoxic drugs. ACKNOWLEDGMENT Grateful thanks are extended to all the physicians who have kindly supplied the information about the patients in this report, to D. A. Bye for carrying out the acyclovir estimation, to Dr. D. S. Freestone and Dr. G. D. W. McKendrick for much help and advice, and especially to Carole Ferdinand0 for tolerance beyond the call of duty in collecting the information and preparing the manuscript.
REFERENCES 1. Tucker WE,Macklin AW, Szot RJ, Clive D: Preclinical safety
2.
evaluation of Zovirax (BW 24811). 18th Interscience Conference on Antimicrobial Agents and Chemotherapy, 1978. Abs. 64. Washington, D.C., American Society for Microbiology. Selby PJ, Powles RL, Jameson B, et al.: Parenteral acylovir
3.
therapy for herpesvirus infections in man. Lancet 1979; II: 1267-1270. Quinn BP, de Miranda P, Gerald L, Good SS: A sensitive radioimmunoassay for the antiviral agent BW248U (9-(2-hydroxyethoxymethyhguanine). Anal Biochem 1979; 98: 319-328.
The American Journal ol Medicine
Acyclovir Sympoelum
185