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The mechanism of action, pharmacokinetics and toxicity of acyclovir — A review
Journal of Infection (1983) 6, Supplement I, 3-9
The m e c h a n i s m of action, p h a r m a c o k i n e t i c s and toxicity o f acyclovir - a r e v i e w D. Brigden and P. Whiteman The Wellcome Research Laboratories, Beckenham, Kent
Summary Acyclovir (ACV) has a novel, highly selective biological activity which results in the inhibition of herpesvirus replication at concentrations 3oo-3ooo-fold lower than those that will inhibit mammalian cellular functions. Subacute and chronic studies in animals indicate that the drug is relatively non-toxic, poses no special risks to pregnancy or the foetus and does not induce detectable oncogenic or genetic changes. In man, acyclovir has a plasma half-life of approximately three hours, is widely distributed throughout the body tissues and is rapidly cleared, mainly as unchanged drug, through the kidneys. Provided that the product (Zovirax®) is used according to the manufacturers' instructions, the risk of significant toxic effects is low.
Introduction T h e discovery of acyclovir (Zovirax ®) was first reported in I 9 7 7 .1 Since then, extensive work has been c o n d u c t e d to establish its m e c h a n i s m of action against the herpes group of viruses, to explore potential toxicity and to evaluate its place in clinical therapeutics. In v i t r o
activity
Acyclovir has been shown to prevent the replication of herpes simplex viruses (HSV) types I and IIfl varicella-zoster (VZ)fl the E p s t e i n - B a r r virus (EBV), 4 cytomegalovirus (CMV) ~ and herpesvirus simiae (B virus of monkeys) 6 in tissue culture. I n general, the replication of H S V is exquisitely sensitive to the d r u g ; EB, VZ and B viruses all show an intermediate sensitivity and C M V is relatively insensitive. It is, however, difficult to draw conclusions of this type from tissue culture experiments as the interaction of the virus and its host cell is crucial w h e n exploring the activity of a drug against a virus. For example, the concentration of acyclovir required to p r o d u c e 50 per cent plaque inhibition of H S V - I strain F9oo4 was 5o-fold lower in h u m a n lung fibroblasts than in green m o n k e y cells. 7
Mechanism of action It has been demonstrated, using radiolabelled acyclovir, that in H S V infected cells, the drug was rapidly metabolised to the m o n o - , di- and triphosphate whereas these were not detectable in uninfected cells. 1 It was also shown that the HSV-specified t h y m i d i n e kinase ( T K ) was responsible for the conversion of acyclovir to its monophosphate. Later, it became clear that cellular guanylate Requests for reprints to D. Brigden.
oi63-4453/83/o3 IOO3 + 07 $o2.oo/o
© 1983 The British Society for the Study of Infection
4
D. B R I G D E N
A N D P. W H I T E M A N
kinase was responsible for the conversion ofmonophosphate to the diphosphate 8 and that several other cellular enzymes could convert the diphosphate to acyclovir triphosphate. Acyclovir has been shown not only to bind 200 times better to the virus T K than the cellular T K but also to be phosphorylated by the viral enzyme three million times faster. 9 Acyclovir triphosphate inhibits HSV-specified D N A polymerase at considerably lower concentrations than cellular D N A polymerase. It is also a much better substrate for the viral than the cellular D N A polymerase; this results in viral D N A chain termination. Furthermore, the D N A terminated by acyclovir monophosphate binds the viral D N A polymerase 50 times more efficiently than the active template. 11 T h e combination of these effects results in a 3oo-3ooo-fold difference between cellular toxicity for HSV. 1 A similar mechanism has been described for the action of acyclovir against VZ virus. 3 T h e EB virus does not code for its own T K but its D N A polymerase is particularly sensitive to inhibition by acyclovir triphosphate; 12 enough acyclovir triphosphate appears to be formed but by mechanisms as yet unknown.
Toxicity studies In a recent publication, 13the in vitro and animal toxicity studies conducted with acyclovir have been reviewed.
(i) Acute and subchronlc toxicity T h e lowest LD5o was seen at 450 m g / k g acyclovir, administered intravenously. It was not possible to achieve LD5os by oral administration. Intravenous doses of 20 m g / k g / d a y and above in rats produced obstructive crystal nephropathy. In dogs, transient deposits of acyclovir crystals were also seen at an intravenous dosage of 25 m g / k g twice daily. Severe toxicity was seen at doses of 50 m g / k g twice daily and above, the effects being essentially radiomimetic and affecting tissues with rapid cell turnover. No adverse effects were seen following oral doses of up to 45o m g / k g for 30 days in mice. Oral dose of 45 m g / k g and above daily, divided into three equal doses, produced vomiting and diarrhoea in dogs, with weight loss after the first week.
(ii) Chronic toxicity A dose of 30 m g / k g per day orally, divided into three equally spaced doses, produced damage to the footpad and nails during the I3th week which recovered despite continuing with the same dose of acyclovir. No other toxic effects were seen during one year's treatment. Oral doses of up to 450 m g / k g / d a y were given to rats; no toxic effects were seen over one year. Similarly, no toxic effects were seen in lifetime studies in mice given acyclovir in doses up to 450 m g / k g / d a y . (iii) Special toxicity No foetal toxicity or teratogenic effects were observed during standard assays in rabbits and rats given doses of up to 25 m g / k g by subcutaneous injection. Mice given doses of 450 m g / k g / d a y by gavage showed no impairment of reproduction or development over two generations.
Pharmacology of acyclovir Extensive evaluation in microbial and mammalian cell assay systems suggest that acyclovir does not damage individual genes or components thereof, even in enormous doses. Nevertheless, chromosomal damage was observed in cultured human lymphocytes exposed to 250 # g / m l acyclovir although no effects were seen at doses of I25 # g / m l and below. T h e level with no effect on chromosomes was, in vitro, about 500 times the HSV ID5o. Mice given intraperitoneal doses of up to 50 m g / k g or oral doses of up to 450 m g / k g survived. No chromosomal damage was seen in Chinese hamsters given doses of IOO mg/kg. Blastogenic effects were seen at doses of 5oo m g / k g and IOOO m g / k g which produced very high blood levels, due to obstructive crystal nephropathy, and a consequent severe and generalized toxicity. Doses of 450 m g / k g given orally to mice and rats over their lifetimes had no effect upon tumour incidence or on life table analysis. T h e conclusions drawn from these extensive toxicity studies are that, provided that obstructive crystal nephropathy is avoided, acyclovir is remarkably non-toxic in short-term and long-term studies and that no particular risks have been identified in the special circumstances of pregnancy or through genetic damage or carcinogenicity. H u m a n pharmacokinetic studies
T h e essential safety demonstrated during the preclinical evaluation encouraged us to start studies in human subjects. These are conducted in normal volunteers and in volunteer patients, either with or at risk of infection with viruses of the herpes group. T h e original studies consisted of administering cautious single oral doses, rising from 5 mg to 6oo mg. T h e drug was well tolerated 1~ so an intravenous study in volunteers was set up. 1~ An infusion of 5 o m g (o'73 mg/kg) was given first, during a one-hour period to mimic the oral plasma profile, then as a Io-minute infusion, and finally as a bolus injection over two minutes. T h e pharmacokinetic parameters calculated from this study are as follows: terminal plasma half-life, 2"9 hours; volume of distribution, 63 1; renal clearance, I85 m l / m i n ; total urine recovery, 73 per cent. Most of the drug was cleared unchanged by the kidney. T h e high renal clearance indicated that tubular secretion as well as glomerular filtration contributed to the excretion of the drug. T h e high volume of distribution suggested that the drug would be distributed throughout the body water. Considerable experience has now been gained from further phase I and II studies in patients with herpesvirus infections. Steady state peak plasma concentrations (CSSmax) after one hour infusions every eight hours were 9"8 _ 2"6 # g / m l and 20- 7 d- io.2 #g/ml, using doses of 5 m g / k g and Io m g / k g respectively. 16 In children, using the equivalent doses of 25o m g / m 2 and 5oo m g / m ~, the CSSmax was I o - 3 _ 4 - o # g / m l and 2o.7#g/ml. 17 T h e mean CSSmax in neonates was I3"8_ 4"I # g / m l in a dosage of Io m g / k g by one hour infusion, is Extensive studies in patients with renal failure have been reported. 19 T h e mean terminal plasma half-life of acyclovir in subjects with end stage renal failure was I9"5 ___5"9 hours. T h e mean plasma half-life during dialysis in the same study was 5"4 hours, the extraction ratio was o'44 and the dialysis
6
Table I
D. B R I G D E N
AND
P. W H I T E M A N
Dosage adjustment recommendedfor patients with renal impairment Creatinine clearance (ml/min/I'75 m2)
Percentage usual dose
Dosing interval (hours)
> 50 25--50 I0"-25 0--I0
I00 IOO IO0 50
8 I2 24 24
clearance was I I3 ml/min. Recommendations17 have been made for dosage adjustment in patients with renal failure (Table I). 14C-acyclovir binding to plasma proteins was I5"4___4"4 per cent. 2° In the same study, approximately 75 per cent of the dose was excreted as unchanged acyclovir and about IO per cent as 9-carboxymethoxymethylguanine. A minor metabolite ( < o.2 per cent of dose) was also identified. Less than two per cent of the radioactivity was excreted in the faeces and only trace amounts in the expired air. Levels of acyclovir in the cerebrospinal fluid are about 50 per cent plasma levels. 17 T h e area under the plasma curve of a 50 mg intravenous dose in six normal volunteers has been shown to be 5"4-fold larger than that following the same dose given by mouth. Oral acyclovir given to normal volunteers as 2oo mg doses four hourly (leaving out the night dose) produced the plasma profile seen in Fig. I. T h e peak plasma level occurred at one to one and a half hours after taking the dose. T h e mean peak for the six individuals varied from o.41 # g / m l to o'77 # g / m l at different times during the course; the overall mean was o'53 #g/ml. Tolerance of acyclovir in man
T h e tolerance of intravenous acyclovir has been carefully monitored during the clinical trial programme in placebo-controlled studies and in patients treated for life threatening disease. A careful evaluation of potential adverse reactions was presented in a recent publication21 Many adverse events were recorded during the treatment of patients with severe life-threatening disease. Some patients died either during or shortly after acyclovir treatment but this has always been associated with the underlying disease or infection and acyclovir has not been implicated. T h e event most clearly related to acyclovir therapy has been a rise in plasma urea or creatinine. This occurred in about i0 per cent of patients during the early use of the drug when acyclovir was given by bolus injection. Information from animal experiments suggests that this phenomenon was due to the deposition of acyclovir crystals in the renal tubules. ~2 This probably occurs when high plasma drug concentrations are being rapidly cleared by the kidneys. T h e risk can largely be avoided by slow intravenous infusion over one hour, maintenance of adequate hydration and the reduction of dosage in patients with impaired renal function (see Table I). Nausea and/or vomiting has been reported in association with high doses (5o0 m g / m 2 eight-hourly intervals) given during
Pharmacology of acyclovir
7
:L
-~z
i' O-
0
I
2 5 Time in days
4
5
Fig. i. Plasma concentration of acyclovir on a regime of 200 m g five times daily for five days. 0 : peak levels I-I½ hours after dose; • : trough levels 4 hours after dose.
a trial of acyclovir in herpes zoster. 23 T h e majority of these patients also had rises in urea and in creatinine which would have been accompained by concomitant rises in plasma acyclovir level. Local reactions occur at the site of infusion when acyclovir at high pH escapes into the surrounding tissues. Occasional reports of neurological reactions have been published. T h e y occurred in four per cent of patients in the early phase II studies. 21 Mild neurological reactions, consisting of tremor and lethargy occurred in two patients receiving high doses of acyclovir for CMV pheumonia. 24 Although the possibility remains that these neurological reactions are associated with acyclovir therapy, it seems more likely that they are associated with the complex underlying problems seen in these patients. Changes in haematological indices have not been consistent and the fact that acyclovir can be administered prophylactically during the whole period from just before a bone-marrow transplant through the period when the grafted marrow is accepted by the host strongly suggests that, at least in doses of up to 25o m g / m 2, the drug does not impair bone marrow function. 25 Indeed there is now good evidence that white-cell counts recover more rapidly in patients treated with acyclovir than those on placebo. 6 Changes in liver enzymes have been difficult to assess, as these rise frequently during herpesvirus infections, even though there is no other evidence of viral infection of the liver. In placebo-controlled trials, rises in liver enzymes have been no more frequent in patients on acyclovir than those on placebo. During trials with oral acyclovir, no adverse reactions have been attributed to the drug. Conclusion
Acyclovir, a novel and highly selective antiviral agent, has proved to have suitable pharmacokinetic properties for successful therapeutic administration. T h e drug has proved safe to handle when used as recommended, both in the context of the seriously ill as well as in those with less life-threatening diseases
8
D. B R I G D E N AND P. W H I T E M A N
s u c h as p r i m a r y g e n i t a l h e r p e s a n d shingles. M u c h w o r k r e m a i n s to b e d o n e to e x p l o r e t h e full role o f this n o v e l t h e r a p e u t i c a g e n t a n d v i g i l a n c e will b e r e q u i r e d to f u r t h e r e v a l u a t e a c y c l o v i r t o l e r a n c e as t h e d r u g b e c o m e s m o r e widely used. (Grateful thanks are extended to Carole Ferdinando for preparing the manuscript.)
References
I. Elion GB, Furman PA, Fyfe JA, De Miranda P, Beauchamp L, Schaeffer HJ. Selectivity of action of an antiherpetic agent, 9-(2-hydroxyethoxymethyl)guanine. Proc Natl Acad Sei USA I977; 74: 5716-572o. 2. Bauer DJ, Collins P, Tucker WE, MacKlin AW. Treatment of experimental Herpes simplex keratitis with Acycloguanosine. Br J Ophthalmol I979; 63: 429-436. 3. Biron KK, Elion GB. In vitro susceptibility ofVaricella zoster virus to Acyclovir. Antimicrob Agents Chemother I98O; I8: 443-447. 4. Colby BM, Shaw JE, Elion GB, Pagano JS. Effect ofacyclovir (9-(2-hydroxyethoxymethyl)guanine). On Epstein-Barr virus DNA replication. J Virol 198o; 34: 560-568. 5. Tyros AS, Scamans EM, Naim HM. The in vitro activity of Acyclovir and related compounds against Cytomegalovirus infections. J A ntimicrob Chemother I981; 8: 65-72. 6. Boulter EA, Thornton B, Bauer DJ, Bye A. Successful treatment of experimental B virus (Herpes virus simiae) infection with Acyclovir. Br M e d J I98O; z8o: 681-688. 7. Harmenberg J, Wahren B. Influence of cell culture conditions on the inhibition of Herpes simplex type I replication by acyclovir. Intervirology 1982; I7:785-791. 8. Miller WH, Miller RL. Phosphorylation of acyclovir monophosphate by GMP kinase. J Biol Chem 198o; 555: 7204-7207. 9- Keller PM, Fyfe JA, Beauchamp L e t al. Enzymic phosphorylation of acyclic nucleoside analogues and correlations with antiherpetic activities. Biochem Pharmacol I98I; 3o: 3o71-3o77. lO. Furman PA, St Clair PA, Fyfe JA, Rideout JL, Keller PM, Elion GB. Inhibition of Herpes simplex virus induced DNA polymerase activity and viral DNA replication by 9-(2hydroxyethoxymethyl) guanine and its triphosphate, ff Virol I979; 35: 72-77. II. Derse D, Chang Y-C, Furman PA, Elion GB. Inhibition of purified human and herpes simplex virus DNA polymerase by 9-(2-hydroxyethoxymethyl)guanine (acyclovir) triphosphate: effects on primer template function. J Biol Chem 1981; 556: I I447-i 1451. I2. Datta AK, Colby BM, Shaw JE, Pagano JS. Acyclovir inhibition of Epstein-Barr virus replication. Proc Natl Acad Sci USA I98o; 77:5163-5 I66. I3. Tucker WE. Preclinical toxicology profile of acyclovir: an overview. Am .7 Med I982; 73 (Suppl IA): 27-30. 14. Brigden D, Fowle A, Rosling A. A new antiherpetic drug: early experience in man with systemically administered drug. In: eds. Collier LH, Oxford J, Developments in antiviral therapy. London : Academic Press, I98O; 52-63. I5. Brigden D, Bye A, Fowle ASE. Human pharmacokinetics of acyclovir (an antiviral agent) following rapid intravenous injection..7 Antimicrob Chemother 1981; 7: 399-404. 16. De Miranda P, Whitley RJ, Blum MR. Acyclovir Kinetics after intravenous infusion. Clin Pharmacol Ther 1979; 56: 718-728. 17. Blum MR, Liao SHT, De Miranda P. Overview of acyclovir pharmacokinetics disposition in adults and children. A m J Med 1982; 73 (Suppl IA): I86-I92. i8. Hintz M, Connor JD, Spector SA, Blum MR, Keeney RE, Yeager AS. Neonatal acyclovir pharmacokinetics in patients with herpes virus infections. Am J Med 1982; 73 (Suppl IA): 2IO--2I 4. r 9. Laskin OL, Longstreth JA, Krasny HC, Keeney RE, Rocco L, Lietman PS. Effect of renal failure on the pharmacokinetics of acyclovir. Am.7 Med I982; 73 (Suppl IA) : 197-2Ol. 20. De Miranda P, Good SS, Laskin OL, Krasny HC, Connor JD, Lietman PS. Disposition of intravenous radioactive acyclovir. Clin Pharmacol Ther I98I ; 30: 662-672.
Pharmacology of acyclovir
9
2 I. Keeney RE, Kirk LE, Brigden D. Acyclovir tolerance in humans. Am J Ailed 1982; 73 (Suppl IA): 176-181. 22. Brigden D, Rosling AE, Woods NC. Renal function after acyclovir intravenous injection. A m J M e d 1982; 73 (Suppl IA): 182-185. 23. Bean B, Bruan C, Balfour HH. Acyclovir therapy for acute herpes zoster. Lancet 1982; ii: 118-121. 24. Wade JC, Hintz M, McGuffm RW, Springmeyer SC, Connor JD, Meyers JD. Treatment of cytomegalovirus pneumonia with high dose acyclovir. Am J Med 1982; 73 (Suppl IA): 249-256. 25. Saral R, Burns WH, Laskin OL, Santos GW, Leitman PS. Acyclovir prophylaxis of herpes simplex virus infections. A randomised double-blind, controlled trial in bone-marrow transplant recipients. New Engl J Med 1981 ; 3o5 : 63-67. 26. Hann IM, Prentice HG, Blacklock HA et al. Acyclovir prophylaxis of herpes virus infections in severely immunocompromised patients: A randomised double blind trial. 2nd International Symposium on Infections in the immunocompromised host. Stifling, Scotland, 20-24 June 1982, No. 142.
The m e c h a n i s m of action, p h a r m a c o k i n e t i c s and toxicity o f acyclovir - a r e v i e w D. Brigden and P. Whiteman The Wellcome Research Laboratories, Beckenham, Kent
Summary Acyclovir (ACV) has a novel, highly selective biological activity which results in the inhibition of herpesvirus replication at concentrations 3oo-3ooo-fold lower than those that will inhibit mammalian cellular functions. Subacute and chronic studies in animals indicate that the drug is relatively non-toxic, poses no special risks to pregnancy or the foetus and does not induce detectable oncogenic or genetic changes. In man, acyclovir has a plasma half-life of approximately three hours, is widely distributed throughout the body tissues and is rapidly cleared, mainly as unchanged drug, through the kidneys. Provided that the product (Zovirax®) is used according to the manufacturers' instructions, the risk of significant toxic effects is low.
Introduction T h e discovery of acyclovir (Zovirax ®) was first reported in I 9 7 7 .1 Since then, extensive work has been c o n d u c t e d to establish its m e c h a n i s m of action against the herpes group of viruses, to explore potential toxicity and to evaluate its place in clinical therapeutics. In v i t r o
activity
Acyclovir has been shown to prevent the replication of herpes simplex viruses (HSV) types I and IIfl varicella-zoster (VZ)fl the E p s t e i n - B a r r virus (EBV), 4 cytomegalovirus (CMV) ~ and herpesvirus simiae (B virus of monkeys) 6 in tissue culture. I n general, the replication of H S V is exquisitely sensitive to the d r u g ; EB, VZ and B viruses all show an intermediate sensitivity and C M V is relatively insensitive. It is, however, difficult to draw conclusions of this type from tissue culture experiments as the interaction of the virus and its host cell is crucial w h e n exploring the activity of a drug against a virus. For example, the concentration of acyclovir required to p r o d u c e 50 per cent plaque inhibition of H S V - I strain F9oo4 was 5o-fold lower in h u m a n lung fibroblasts than in green m o n k e y cells. 7
Mechanism of action It has been demonstrated, using radiolabelled acyclovir, that in H S V infected cells, the drug was rapidly metabolised to the m o n o - , di- and triphosphate whereas these were not detectable in uninfected cells. 1 It was also shown that the HSV-specified t h y m i d i n e kinase ( T K ) was responsible for the conversion of acyclovir to its monophosphate. Later, it became clear that cellular guanylate Requests for reprints to D. Brigden.
oi63-4453/83/o3 IOO3 + 07 $o2.oo/o
© 1983 The British Society for the Study of Infection
4
D. B R I G D E N
A N D P. W H I T E M A N
kinase was responsible for the conversion ofmonophosphate to the diphosphate 8 and that several other cellular enzymes could convert the diphosphate to acyclovir triphosphate. Acyclovir has been shown not only to bind 200 times better to the virus T K than the cellular T K but also to be phosphorylated by the viral enzyme three million times faster. 9 Acyclovir triphosphate inhibits HSV-specified D N A polymerase at considerably lower concentrations than cellular D N A polymerase. It is also a much better substrate for the viral than the cellular D N A polymerase; this results in viral D N A chain termination. Furthermore, the D N A terminated by acyclovir monophosphate binds the viral D N A polymerase 50 times more efficiently than the active template. 11 T h e combination of these effects results in a 3oo-3ooo-fold difference between cellular toxicity for HSV. 1 A similar mechanism has been described for the action of acyclovir against VZ virus. 3 T h e EB virus does not code for its own T K but its D N A polymerase is particularly sensitive to inhibition by acyclovir triphosphate; 12 enough acyclovir triphosphate appears to be formed but by mechanisms as yet unknown.
Toxicity studies In a recent publication, 13the in vitro and animal toxicity studies conducted with acyclovir have been reviewed.
(i) Acute and subchronlc toxicity T h e lowest LD5o was seen at 450 m g / k g acyclovir, administered intravenously. It was not possible to achieve LD5os by oral administration. Intravenous doses of 20 m g / k g / d a y and above in rats produced obstructive crystal nephropathy. In dogs, transient deposits of acyclovir crystals were also seen at an intravenous dosage of 25 m g / k g twice daily. Severe toxicity was seen at doses of 50 m g / k g twice daily and above, the effects being essentially radiomimetic and affecting tissues with rapid cell turnover. No adverse effects were seen following oral doses of up to 45o m g / k g for 30 days in mice. Oral dose of 45 m g / k g and above daily, divided into three equal doses, produced vomiting and diarrhoea in dogs, with weight loss after the first week.
(ii) Chronic toxicity A dose of 30 m g / k g per day orally, divided into three equally spaced doses, produced damage to the footpad and nails during the I3th week which recovered despite continuing with the same dose of acyclovir. No other toxic effects were seen during one year's treatment. Oral doses of up to 450 m g / k g / d a y were given to rats; no toxic effects were seen over one year. Similarly, no toxic effects were seen in lifetime studies in mice given acyclovir in doses up to 450 m g / k g / d a y . (iii) Special toxicity No foetal toxicity or teratogenic effects were observed during standard assays in rabbits and rats given doses of up to 25 m g / k g by subcutaneous injection. Mice given doses of 450 m g / k g / d a y by gavage showed no impairment of reproduction or development over two generations.
Pharmacology of acyclovir Extensive evaluation in microbial and mammalian cell assay systems suggest that acyclovir does not damage individual genes or components thereof, even in enormous doses. Nevertheless, chromosomal damage was observed in cultured human lymphocytes exposed to 250 # g / m l acyclovir although no effects were seen at doses of I25 # g / m l and below. T h e level with no effect on chromosomes was, in vitro, about 500 times the HSV ID5o. Mice given intraperitoneal doses of up to 50 m g / k g or oral doses of up to 450 m g / k g survived. No chromosomal damage was seen in Chinese hamsters given doses of IOO mg/kg. Blastogenic effects were seen at doses of 5oo m g / k g and IOOO m g / k g which produced very high blood levels, due to obstructive crystal nephropathy, and a consequent severe and generalized toxicity. Doses of 450 m g / k g given orally to mice and rats over their lifetimes had no effect upon tumour incidence or on life table analysis. T h e conclusions drawn from these extensive toxicity studies are that, provided that obstructive crystal nephropathy is avoided, acyclovir is remarkably non-toxic in short-term and long-term studies and that no particular risks have been identified in the special circumstances of pregnancy or through genetic damage or carcinogenicity. H u m a n pharmacokinetic studies
T h e essential safety demonstrated during the preclinical evaluation encouraged us to start studies in human subjects. These are conducted in normal volunteers and in volunteer patients, either with or at risk of infection with viruses of the herpes group. T h e original studies consisted of administering cautious single oral doses, rising from 5 mg to 6oo mg. T h e drug was well tolerated 1~ so an intravenous study in volunteers was set up. 1~ An infusion of 5 o m g (o'73 mg/kg) was given first, during a one-hour period to mimic the oral plasma profile, then as a Io-minute infusion, and finally as a bolus injection over two minutes. T h e pharmacokinetic parameters calculated from this study are as follows: terminal plasma half-life, 2"9 hours; volume of distribution, 63 1; renal clearance, I85 m l / m i n ; total urine recovery, 73 per cent. Most of the drug was cleared unchanged by the kidney. T h e high renal clearance indicated that tubular secretion as well as glomerular filtration contributed to the excretion of the drug. T h e high volume of distribution suggested that the drug would be distributed throughout the body water. Considerable experience has now been gained from further phase I and II studies in patients with herpesvirus infections. Steady state peak plasma concentrations (CSSmax) after one hour infusions every eight hours were 9"8 _ 2"6 # g / m l and 20- 7 d- io.2 #g/ml, using doses of 5 m g / k g and Io m g / k g respectively. 16 In children, using the equivalent doses of 25o m g / m 2 and 5oo m g / m ~, the CSSmax was I o - 3 _ 4 - o # g / m l and 2o.7#g/ml. 17 T h e mean CSSmax in neonates was I3"8_ 4"I # g / m l in a dosage of Io m g / k g by one hour infusion, is Extensive studies in patients with renal failure have been reported. 19 T h e mean terminal plasma half-life of acyclovir in subjects with end stage renal failure was I9"5 ___5"9 hours. T h e mean plasma half-life during dialysis in the same study was 5"4 hours, the extraction ratio was o'44 and the dialysis
6
Table I
D. B R I G D E N
AND
P. W H I T E M A N
Dosage adjustment recommendedfor patients with renal impairment Creatinine clearance (ml/min/I'75 m2)
Percentage usual dose
Dosing interval (hours)
> 50 25--50 I0"-25 0--I0
I00 IOO IO0 50
8 I2 24 24
clearance was I I3 ml/min. Recommendations17 have been made for dosage adjustment in patients with renal failure (Table I). 14C-acyclovir binding to plasma proteins was I5"4___4"4 per cent. 2° In the same study, approximately 75 per cent of the dose was excreted as unchanged acyclovir and about IO per cent as 9-carboxymethoxymethylguanine. A minor metabolite ( < o.2 per cent of dose) was also identified. Less than two per cent of the radioactivity was excreted in the faeces and only trace amounts in the expired air. Levels of acyclovir in the cerebrospinal fluid are about 50 per cent plasma levels. 17 T h e area under the plasma curve of a 50 mg intravenous dose in six normal volunteers has been shown to be 5"4-fold larger than that following the same dose given by mouth. Oral acyclovir given to normal volunteers as 2oo mg doses four hourly (leaving out the night dose) produced the plasma profile seen in Fig. I. T h e peak plasma level occurred at one to one and a half hours after taking the dose. T h e mean peak for the six individuals varied from o.41 # g / m l to o'77 # g / m l at different times during the course; the overall mean was o'53 #g/ml. Tolerance of acyclovir in man
T h e tolerance of intravenous acyclovir has been carefully monitored during the clinical trial programme in placebo-controlled studies and in patients treated for life threatening disease. A careful evaluation of potential adverse reactions was presented in a recent publication21 Many adverse events were recorded during the treatment of patients with severe life-threatening disease. Some patients died either during or shortly after acyclovir treatment but this has always been associated with the underlying disease or infection and acyclovir has not been implicated. T h e event most clearly related to acyclovir therapy has been a rise in plasma urea or creatinine. This occurred in about i0 per cent of patients during the early use of the drug when acyclovir was given by bolus injection. Information from animal experiments suggests that this phenomenon was due to the deposition of acyclovir crystals in the renal tubules. ~2 This probably occurs when high plasma drug concentrations are being rapidly cleared by the kidneys. T h e risk can largely be avoided by slow intravenous infusion over one hour, maintenance of adequate hydration and the reduction of dosage in patients with impaired renal function (see Table I). Nausea and/or vomiting has been reported in association with high doses (5o0 m g / m 2 eight-hourly intervals) given during
Pharmacology of acyclovir
7
:L
-~z
i' O-
0
I
2 5 Time in days
4
5
Fig. i. Plasma concentration of acyclovir on a regime of 200 m g five times daily for five days. 0 : peak levels I-I½ hours after dose; • : trough levels 4 hours after dose.
a trial of acyclovir in herpes zoster. 23 T h e majority of these patients also had rises in urea and in creatinine which would have been accompained by concomitant rises in plasma acyclovir level. Local reactions occur at the site of infusion when acyclovir at high pH escapes into the surrounding tissues. Occasional reports of neurological reactions have been published. T h e y occurred in four per cent of patients in the early phase II studies. 21 Mild neurological reactions, consisting of tremor and lethargy occurred in two patients receiving high doses of acyclovir for CMV pheumonia. 24 Although the possibility remains that these neurological reactions are associated with acyclovir therapy, it seems more likely that they are associated with the complex underlying problems seen in these patients. Changes in haematological indices have not been consistent and the fact that acyclovir can be administered prophylactically during the whole period from just before a bone-marrow transplant through the period when the grafted marrow is accepted by the host strongly suggests that, at least in doses of up to 25o m g / m 2, the drug does not impair bone marrow function. 25 Indeed there is now good evidence that white-cell counts recover more rapidly in patients treated with acyclovir than those on placebo. 6 Changes in liver enzymes have been difficult to assess, as these rise frequently during herpesvirus infections, even though there is no other evidence of viral infection of the liver. In placebo-controlled trials, rises in liver enzymes have been no more frequent in patients on acyclovir than those on placebo. During trials with oral acyclovir, no adverse reactions have been attributed to the drug. Conclusion
Acyclovir, a novel and highly selective antiviral agent, has proved to have suitable pharmacokinetic properties for successful therapeutic administration. T h e drug has proved safe to handle when used as recommended, both in the context of the seriously ill as well as in those with less life-threatening diseases
8
D. B R I G D E N AND P. W H I T E M A N
s u c h as p r i m a r y g e n i t a l h e r p e s a n d shingles. M u c h w o r k r e m a i n s to b e d o n e to e x p l o r e t h e full role o f this n o v e l t h e r a p e u t i c a g e n t a n d v i g i l a n c e will b e r e q u i r e d to f u r t h e r e v a l u a t e a c y c l o v i r t o l e r a n c e as t h e d r u g b e c o m e s m o r e widely used. (Grateful thanks are extended to Carole Ferdinando for preparing the manuscript.)
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