126
4th F O R U M I N V I R O L O G Y
occur, and since chronic therapy with foscarnet cardes both the risk of indwelling-catheter-related complications and of the emergence of resistance to foscarnet. The option of chronic topical prophylaxis appears unlikely to succeed, given the contiguous nature of ganglionic involvement, as well as the likelihood of development of resistance locally. Unfortunately, a sample size calculation indicates that a total of 153 patients would be required for a three-arm trial which compares acyclovir chemosuppression to maintenance foscarnet therapy to no antiherpes suppressive therapy following healing of a herpetic lesion which is resistant to acyclovir, in order to be able to detect a doubling in the time to acyclovir-resistant recurrence of HSV with a twotailed level of significance of 0.05 and a power of 80 %. This large sample size will, in all likelihood, make it unfeasible to perform the definitive study.
References Clements, G.B. & Subak-Sharpe, J.H. (1988), Herpes simplex type 2 establishes latency in the mouse footpad. J. gen. Virol., 69, 375-383. Coen, D.M., Kosz-Vnenchak, M., Jacobson, J.G. et al. (1989), Thymidine kinase-negative herpes simplex virus mutants establish latency in mouse trigeminal ganglia but do not reactivate. Proc. nat. Acad. Sci. (Wash.), 86, 4736-4740. Engel, J.P., Englund, J.A., Fletcher, C.V. & Hill, E.L.
(1990), Treatment of resistant herpes simplex virus with continuous-infusion acyclovir. J. Amer. med. Ass., 263, 1662-1664. Erlich, K.S., Mills, J., Chatis, P. et aL (1989), Acyclovirresistant herpes simplex virus infections in patients with acquired immunodeficiency syndrome. New Engl. J. Med., 320, 293-296. Field, H.J. & Darby, G. (1980), Pathogenicity in mice of strains of herpes simplex virus which are resistant to acyclovir in vitro and in vivo. Antimicrob. Agents a. Chemother., 17, 209-216. Safrin, S., Assaykeen, T., FoUansbee, S. & Mills, J. (1990), Foscaruet therapy for acyclovir-resistant mucocutaneous herpes simplex virus infection in 26 AIDS patients: preliminary data. J. infect. D/s., 181, 1078-1084. Safrin, S., Crumpacker, C., Chatis, P. et al. (1991), A controlled trial comparing foscarnet with vidarabine for acyclovir-resistant mucocutaneous herpes simplex in the acquired immunodeficiency syndrome. New Engl. J. Med., 325, 551-555. Scriba, M. (1977), Extraneural localisation of herpes simplex virus in latently infected guinea pigs. Nature (Lond.), 267, 529-531. Stanberry, L.R., Kit, S. & Myers, M.G. (1985), Thymidine kinase-deficient herpes simplex virus type 2 genital infection in guinea pigs. J. Virol., 55,322-328. Stevens, J.G. (1989), Human herpesviruses: a consideration of the latent state. Microbioi. Rev., 53, 318-332. Weaver, D., Weissbach, N., Kapell, K., Benson, C.A., Pottage, J.C. & Kessler, H.A. (1991), Topical trifuridine (TFT) treatment of acyclovir-resistant herpes simplex disease. Thirty-first Interscience Conference. Antimicrob. Agents a. Chemother. (Abstract 507).
The clinical significance of in vitro cytomegalovirus susceptibility to antiviral drugs J.-M. P6pin, F. Simon, M.C. Dazza and F. Brun-V6zinet Virology Department, H6pital Bichat/Claude Bernard, 46 rue H e n r i Huchard, 75018 Paris
As in the case of herpes simpl~ and acyclovir, CMV resistance to ganciclovir (GCV) was initially observed in a laboratory strain (Biron et al., 1986). This strain exhibited a wild-type susceptibility to inhibitors of CMV DNA polymerase, such as foscarnet (PFA). The resistance was linked to a reduction of
GCV phosphorylation in infected cells, suggesting that CMV encodes a nucleoside kinase (or a gene product that induces a cellular kinase) which is probably deficient or altered in this strain. Erice et al. (1989) and Drew et al. (1991) obtained GCV-resistant (GCV-R) PFA-sensitive (PFA-S) clinical isolates
R E S I S T A N C E TO A N T I V I R A L
which presented a similar mechanism of resistance (Biron, 1991). PFA-R/GCV-S laboratory strains - probably with altered DNA polymerase - - were recently observed (Sullivan and Coen, 1991) and the isolation of a clinical isolate resistant to PFA and GCV (Knox et ai., 1991) needs to be confirmed.
What is the clinical significance of GCV-resistant CMV isolates?
To date, the number of GCV-R isolates described is inadequate to validate a correlation between in vitro susceptibility data and the clinical course. Nevertheless, this correlation seems striking in the study reported by Erice et ai. (1989): three GCV-R CMV (one of them being resistant prior to any GCV therapy) were recovered from patients presenting with progressive CMV disease which was clinically resistant to GCV. In the study reported by Drew et al. (1991), among 13 isolates obtained from AIDS patients treated for /> 3 months with GCV, 5 isolates were considered to be GCV-R and were associated with progression of CMV retinitis. These studies clearly demonstrated that GCV-R CMV were not altered in pathogenicity and that the emergence of GCV-R variants, with an estimated prevalence of 7.6 °70 in patients taking GCV for > 3 months (Drew et al., 1991), is a source of concern. Regarding the clinical significance of in vitro resistance, it is possible that an in vitro resistant isolate responding favourably to antiviral treatment or a sensitive isolate from a patient presenting with clinical resistance may be of lesser interest for publication than resistant isolates escaping therapy. A study which rigorously evaluates the clinical significance of sensitive versus resistant CMV isolates in GCVtreated patients is yet to be performed.
What is the definition of the in vitro resistance ?
As the genetic basis of GCV resistance is still unknown, only phenotypic studies can be performed at present. Two studies performed on a large number (54 isolates in both studies) of clinical isolates tested with the same plaque reduction assay showed that wild sensitive strains exhibit a wide range of susceptibility, with IC50 values ranging from 0.65 to 6.3 ~M (Cole and Balfour, 1987) and from 0.4 to 11 ~tM (Plotkin et al., 1985), i.e. more than a 10-fold variation. The values of ICs0 and ICgo may differ with the assay system (plaque reduction (Cheng et al., 1983 ; Mar et aL, 1983), yield reduction (Rasmussen et al., 1984), late antigen synthesis reduction
DRUGS
127
(Mazeron et al., 1989), DNA hybridization (Dunker et al., 1990)). However, even with the same assay (plaque reduction) performed on the same cells (MRC-5) with the same strain (ADI69), ICs0 values may vary from laboratory to laboratory, from 1.6 ~M (Field et al., 1983) to 7 + 3 ~,1 (Freitas et al., 1983). GCV-R CMV generally do not exhibit a dramatic increase in IC50 values. For example, Drew et al. (1991) reported Its0 values of 4.6 and 4.8 I~M (mean of GCV-sensitive isolates = 3.2 IxM) in two GCV-R isolates; their ICgo were about 3 and 6 times those of sensitive control strains. Using the same plaque reduction assay, the highest ICs0 value reported by Erice et ai. (1989) was 30.5 ~M (6.4-fold the value of control strains), with a spectacular increase in ICg0 (35 times those of the controls). These results suggest that ICgo values could be more indicative of resistance than ICs0 but, in practice, ICgo is less precise and reproducible than the ICs0. It is thus very difficult, even for a consensus standardized test, to set an absolute ICs0 (or ICgo) threshold value for in vitro resistance. The comparison between an early (prior to therapy) and a late isolate tested simultaneously, demonstrating a significant (this term is yet to be defined) increase in ICs0 and/or ICg0 values, obviously represents the best criterion of the late isolate resistance. In practice, the early isolate is not always available and the laboratory has to give a verdict on a sole isolate. To define the in vitro resistance of this isolate, with a definition suitable for every laboratory and every assay, the isolate might be simultaneously tested with a reference-standardized sensitive strain (ADI69 strain for example). In vitro resistance might then be defined as an IC50 and/or an IC90 value significantly higher than that observed for the AD169 strain simultaneously tested. The significance level needs to be defined: 2-fold, 4-fold higher? In our laboratory, we obtained two isolates from HIV patients exhibiting IC50 values of 10.8 and I0.5 ~tM, respectively, i.e. 2.15- and 2.7-fold higher than the ICs0 of AD169 simultaneously tested. It was difficult to consider these isolates as resistant: they were obtained from GCV-naive patients with an excellent clinical response to GCV therapy (unpublished data). In our experience, using a test with serial 3-fold dilutions of GCV, a 3-fold increase in ICs0 and/or ICg0 seemed significant. Since an excellent correlation exists between ICs0 values and the inability to induce GCV triphosphate in infected cells (Biron, 1991), this biochemical approach might be proposed to validate the susceptibility assay. Unfortunately, it is not suitable for PFA studies and cannot detect other potential mechanisms of GCV resistance as altered DNA polymerase.
128
4th F O R U M I N V I R O L O G Y
What is the clinical significance o f GCV-sensitive isolates ?
If the isolate is considered as in vitro sensitive to GCV, can this predict a favourable clinical outcome with GCV therapy ? In vitro sensitivity is obviously not synonymous with therapeutic success for at least two reasons. First, the clinical response to every antibiotic or antiviral therapy is related to interindividual variations of compliance to therapy and pharmacokinetic parameters - - including intracellular diffusion m and, above all, to the possibility that the treatment is administered early in the clinical course. The second reason is more peculiar to CMV drug sensitivity tests. The existing assays require the isolation of CMV from clinical specimens and the isolate has to be passaged several times (generally I> 5) in cell culture to increase the titre of virus a n d / o r to obtain cell-free virus before testing. During this procedure there may be reversion from the GCV-R to the GCV-S phenotype or the selection of GCV-S strains from a mixed GCV-S/GCV-R population in the clinical sample. The phenotypic susceptibility assay would then be performed on a selected or reverted strain which is not representative of the in vivo population. An approach to the problem might be to perform a susceptibility test directly on primocultures of clinical specimens with observation of the results after a single replication cycle of the virus. A late antigen synthesis reduction system (Mazeron et al., 1989) allows the practical application of this approach and, moreover, provides the result as early as 5 days after receiving the specimen (P~pin et al., manuscript in preparation). In conclusion, the emergence of CMV resistance to drugs is not a rare event, especially in patients on long-term anti-CMV therapy. Rigourous evaluation of the clinical significance of sensitive versus resistant CMV isolates, with a standardized definition of the in vitro resistance, needs to be performed. Rapid susceptibility tests are needed, since their results could play a direct role in the therapeutic decision.
References
Biron, K.K. (1991), Ganciclovir-resistant human cytomegalovirus clinical isolates; resistance mechanisms and in vitro susceptibility to antiviral agents. Transplant. Proc., 23 (suppl. 3), 162-167. Biron, K.K., Fyfe, J.A., Stanat, S.C. et al. (1986), A human cytomegalovirus mutant resistant to the nucleoside analog 9-[[2-hydroxy-l-(hydroxymethyl)ethoxy]methyl]-guanine (BW B759U) induces
reduced levels of BW B759U triphosphate. Proc. nat. Acad. Sci. (Wash.), 83, 8769-73. Cheng, Y.C., Huang, E.S., Lin, J.C. et al. (1983), Unique spectrum of activity of 9-[(l,3-dihydroxy-2-propoxy)methyl]-guanine against herpes viruses in vitro and its mode of action against herpes simplex virus type 1. Proc. nat. Acad. Sci. (Wash.), 80, 2767-2770. Cole, N.L. & Balfour, H.H. Jr. (1987), In vitro susceptibility of cytomegalovirus isolates from immunocompromised patients to acyclovir and ganciclovir. Diagn. Microbiol. infect. D/s., 6, 255-261. Dankner, W.M., Scholl, D., Stanat, S.C. et al. (1990), Rapid antiviral DNA-DNA hybridization assay for human cytomegalovirus. J. Virol. Methods, 28, 293 -298. Drew, W.L., Miner, R. & King, D. (1991), Antiviral activity of FIAU (1-[2'-deoxy-2'-fluoro-l-[3-d-arabinofuranosyl-]-5-iodo-uridine) on strains of cytomegalovirus sensitive and resistant to ganciclovir. J. infect. Dis., 163, 1389-1390. Drew, W.L., Miner, R.C., Bush, D.F. et al. (1991), Prevalence of resistance in patients receiving ganciclovir for serious cytomegalovirus infection. J. infect. Dis., 163, 716-719. Erice, A., Chou, S., Biron, K.K. et al. (1989), Progressive disease due to ganciclovir-resistant cytomegalovirus in immunocompromised patients. New Engl. J. Med., 320, 289-293. Field, A.K., Davies, M.E., De Witt, C. et al. (1983), 9-([2-hydroxy- l-(hydroxymethyl)ethoxy]methyl)-guanine: a selective inhibitor of herpes group virus replication. Proc. nat. Acad. Sci. (Wash.), 80, 4139-4143. Freitas, V.R., Smee, D.F., Chernow, M. et al. (1985), Activity of 9-(l,3-dihydroxy-2-propoxymethyl)guanine compared with that of acyclovir against human, monkey and rodent cytomegaloviruses. Antimicrob. Agents a. Chemother., 28, 240-245. Knox, K.K., Drobyski, W.R. & Carrigan, D.R. (1991), Cytomegaiovirus isolate resistant to ganciclovir and foscarnet from a marrow transplant patient. Lancet, II, 1292-1293. Mar, E.C., Cheng, Y.T., Huang, E.S. et al. (1983), Effect of 9-(l,3-dihydroxy-2-propoxymethyl)guanine on human cytomegalovirus replication in vitro. Antimicrob. Agents a. Chemother., 24, 518-521. Mazeron, M.C., St-Jean, L.A., Defer, M.C., Gluckman, E., Nebout, T. & P6rol, Y. (1989), Rapid method for determining susceptibility of human cytomegalovirus strains to antiviral drugs. IInd International Cytomegalovirus Worshop, San Diego, USA. Plotkin, S.A., Drew, W.L., Felsenstein, D. et al. (1985), Sensitivity of clinical isolates of human cytomegalovirus to 9-(l,3-dihydroxy-2-propoxymethyl)-guanine. J. infect. Dis., I52, 833-834. Rasmussen, L., Chen, P.T., Mullenax, J.G. et al. (1984), Inhibition of human cytomegalovirus replication by 9-(1,3-dihydroxy-2-propoxymethyl)-guanine alone and in combination with human interferons. Antimicrob. Agents a. Chemother., 26, 441-445. Sullivan, V. & Coen, D.M. (1991), Isolation of foscarnetresistant human cytomegalovirus patterns of resistance and sensitivity to other antiviral drugs. J. infect. Dis., 164, 781-784.
4th F O R U M I N V I R O L O G Y
occur, and since chronic therapy with foscarnet cardes both the risk of indwelling-catheter-related complications and of the emergence of resistance to foscarnet. The option of chronic topical prophylaxis appears unlikely to succeed, given the contiguous nature of ganglionic involvement, as well as the likelihood of development of resistance locally. Unfortunately, a sample size calculation indicates that a total of 153 patients would be required for a three-arm trial which compares acyclovir chemosuppression to maintenance foscarnet therapy to no antiherpes suppressive therapy following healing of a herpetic lesion which is resistant to acyclovir, in order to be able to detect a doubling in the time to acyclovir-resistant recurrence of HSV with a twotailed level of significance of 0.05 and a power of 80 %. This large sample size will, in all likelihood, make it unfeasible to perform the definitive study.
References Clements, G.B. & Subak-Sharpe, J.H. (1988), Herpes simplex type 2 establishes latency in the mouse footpad. J. gen. Virol., 69, 375-383. Coen, D.M., Kosz-Vnenchak, M., Jacobson, J.G. et al. (1989), Thymidine kinase-negative herpes simplex virus mutants establish latency in mouse trigeminal ganglia but do not reactivate. Proc. nat. Acad. Sci. (Wash.), 86, 4736-4740. Engel, J.P., Englund, J.A., Fletcher, C.V. & Hill, E.L.
(1990), Treatment of resistant herpes simplex virus with continuous-infusion acyclovir. J. Amer. med. Ass., 263, 1662-1664. Erlich, K.S., Mills, J., Chatis, P. et aL (1989), Acyclovirresistant herpes simplex virus infections in patients with acquired immunodeficiency syndrome. New Engl. J. Med., 320, 293-296. Field, H.J. & Darby, G. (1980), Pathogenicity in mice of strains of herpes simplex virus which are resistant to acyclovir in vitro and in vivo. Antimicrob. Agents a. Chemother., 17, 209-216. Safrin, S., Assaykeen, T., FoUansbee, S. & Mills, J. (1990), Foscaruet therapy for acyclovir-resistant mucocutaneous herpes simplex virus infection in 26 AIDS patients: preliminary data. J. infect. D/s., 181, 1078-1084. Safrin, S., Crumpacker, C., Chatis, P. et al. (1991), A controlled trial comparing foscarnet with vidarabine for acyclovir-resistant mucocutaneous herpes simplex in the acquired immunodeficiency syndrome. New Engl. J. Med., 325, 551-555. Scriba, M. (1977), Extraneural localisation of herpes simplex virus in latently infected guinea pigs. Nature (Lond.), 267, 529-531. Stanberry, L.R., Kit, S. & Myers, M.G. (1985), Thymidine kinase-deficient herpes simplex virus type 2 genital infection in guinea pigs. J. Virol., 55,322-328. Stevens, J.G. (1989), Human herpesviruses: a consideration of the latent state. Microbioi. Rev., 53, 318-332. Weaver, D., Weissbach, N., Kapell, K., Benson, C.A., Pottage, J.C. & Kessler, H.A. (1991), Topical trifuridine (TFT) treatment of acyclovir-resistant herpes simplex disease. Thirty-first Interscience Conference. Antimicrob. Agents a. Chemother. (Abstract 507).
The clinical significance of in vitro cytomegalovirus susceptibility to antiviral drugs J.-M. P6pin, F. Simon, M.C. Dazza and F. Brun-V6zinet Virology Department, H6pital Bichat/Claude Bernard, 46 rue H e n r i Huchard, 75018 Paris
As in the case of herpes simpl~ and acyclovir, CMV resistance to ganciclovir (GCV) was initially observed in a laboratory strain (Biron et al., 1986). This strain exhibited a wild-type susceptibility to inhibitors of CMV DNA polymerase, such as foscarnet (PFA). The resistance was linked to a reduction of
GCV phosphorylation in infected cells, suggesting that CMV encodes a nucleoside kinase (or a gene product that induces a cellular kinase) which is probably deficient or altered in this strain. Erice et al. (1989) and Drew et al. (1991) obtained GCV-resistant (GCV-R) PFA-sensitive (PFA-S) clinical isolates
R E S I S T A N C E TO A N T I V I R A L
which presented a similar mechanism of resistance (Biron, 1991). PFA-R/GCV-S laboratory strains - probably with altered DNA polymerase - - were recently observed (Sullivan and Coen, 1991) and the isolation of a clinical isolate resistant to PFA and GCV (Knox et ai., 1991) needs to be confirmed.
What is the clinical significance of GCV-resistant CMV isolates?
To date, the number of GCV-R isolates described is inadequate to validate a correlation between in vitro susceptibility data and the clinical course. Nevertheless, this correlation seems striking in the study reported by Erice et ai. (1989): three GCV-R CMV (one of them being resistant prior to any GCV therapy) were recovered from patients presenting with progressive CMV disease which was clinically resistant to GCV. In the study reported by Drew et al. (1991), among 13 isolates obtained from AIDS patients treated for /> 3 months with GCV, 5 isolates were considered to be GCV-R and were associated with progression of CMV retinitis. These studies clearly demonstrated that GCV-R CMV were not altered in pathogenicity and that the emergence of GCV-R variants, with an estimated prevalence of 7.6 °70 in patients taking GCV for > 3 months (Drew et al., 1991), is a source of concern. Regarding the clinical significance of in vitro resistance, it is possible that an in vitro resistant isolate responding favourably to antiviral treatment or a sensitive isolate from a patient presenting with clinical resistance may be of lesser interest for publication than resistant isolates escaping therapy. A study which rigorously evaluates the clinical significance of sensitive versus resistant CMV isolates in GCVtreated patients is yet to be performed.
What is the definition of the in vitro resistance ?
As the genetic basis of GCV resistance is still unknown, only phenotypic studies can be performed at present. Two studies performed on a large number (54 isolates in both studies) of clinical isolates tested with the same plaque reduction assay showed that wild sensitive strains exhibit a wide range of susceptibility, with IC50 values ranging from 0.65 to 6.3 ~M (Cole and Balfour, 1987) and from 0.4 to 11 ~tM (Plotkin et al., 1985), i.e. more than a 10-fold variation. The values of ICs0 and ICgo may differ with the assay system (plaque reduction (Cheng et al., 1983 ; Mar et aL, 1983), yield reduction (Rasmussen et al., 1984), late antigen synthesis reduction
DRUGS
127
(Mazeron et al., 1989), DNA hybridization (Dunker et al., 1990)). However, even with the same assay (plaque reduction) performed on the same cells (MRC-5) with the same strain (ADI69), ICs0 values may vary from laboratory to laboratory, from 1.6 ~M (Field et al., 1983) to 7 + 3 ~,1 (Freitas et al., 1983). GCV-R CMV generally do not exhibit a dramatic increase in IC50 values. For example, Drew et al. (1991) reported Its0 values of 4.6 and 4.8 I~M (mean of GCV-sensitive isolates = 3.2 IxM) in two GCV-R isolates; their ICgo were about 3 and 6 times those of sensitive control strains. Using the same plaque reduction assay, the highest ICs0 value reported by Erice et ai. (1989) was 30.5 ~M (6.4-fold the value of control strains), with a spectacular increase in ICg0 (35 times those of the controls). These results suggest that ICgo values could be more indicative of resistance than ICs0 but, in practice, ICgo is less precise and reproducible than the ICs0. It is thus very difficult, even for a consensus standardized test, to set an absolute ICs0 (or ICgo) threshold value for in vitro resistance. The comparison between an early (prior to therapy) and a late isolate tested simultaneously, demonstrating a significant (this term is yet to be defined) increase in ICs0 and/or ICg0 values, obviously represents the best criterion of the late isolate resistance. In practice, the early isolate is not always available and the laboratory has to give a verdict on a sole isolate. To define the in vitro resistance of this isolate, with a definition suitable for every laboratory and every assay, the isolate might be simultaneously tested with a reference-standardized sensitive strain (ADI69 strain for example). In vitro resistance might then be defined as an IC50 and/or an IC90 value significantly higher than that observed for the AD169 strain simultaneously tested. The significance level needs to be defined: 2-fold, 4-fold higher? In our laboratory, we obtained two isolates from HIV patients exhibiting IC50 values of 10.8 and I0.5 ~tM, respectively, i.e. 2.15- and 2.7-fold higher than the ICs0 of AD169 simultaneously tested. It was difficult to consider these isolates as resistant: they were obtained from GCV-naive patients with an excellent clinical response to GCV therapy (unpublished data). In our experience, using a test with serial 3-fold dilutions of GCV, a 3-fold increase in ICs0 and/or ICg0 seemed significant. Since an excellent correlation exists between ICs0 values and the inability to induce GCV triphosphate in infected cells (Biron, 1991), this biochemical approach might be proposed to validate the susceptibility assay. Unfortunately, it is not suitable for PFA studies and cannot detect other potential mechanisms of GCV resistance as altered DNA polymerase.
128
4th F O R U M I N V I R O L O G Y
What is the clinical significance o f GCV-sensitive isolates ?
If the isolate is considered as in vitro sensitive to GCV, can this predict a favourable clinical outcome with GCV therapy ? In vitro sensitivity is obviously not synonymous with therapeutic success for at least two reasons. First, the clinical response to every antibiotic or antiviral therapy is related to interindividual variations of compliance to therapy and pharmacokinetic parameters - - including intracellular diffusion m and, above all, to the possibility that the treatment is administered early in the clinical course. The second reason is more peculiar to CMV drug sensitivity tests. The existing assays require the isolation of CMV from clinical specimens and the isolate has to be passaged several times (generally I> 5) in cell culture to increase the titre of virus a n d / o r to obtain cell-free virus before testing. During this procedure there may be reversion from the GCV-R to the GCV-S phenotype or the selection of GCV-S strains from a mixed GCV-S/GCV-R population in the clinical sample. The phenotypic susceptibility assay would then be performed on a selected or reverted strain which is not representative of the in vivo population. An approach to the problem might be to perform a susceptibility test directly on primocultures of clinical specimens with observation of the results after a single replication cycle of the virus. A late antigen synthesis reduction system (Mazeron et al., 1989) allows the practical application of this approach and, moreover, provides the result as early as 5 days after receiving the specimen (P~pin et al., manuscript in preparation). In conclusion, the emergence of CMV resistance to drugs is not a rare event, especially in patients on long-term anti-CMV therapy. Rigourous evaluation of the clinical significance of sensitive versus resistant CMV isolates, with a standardized definition of the in vitro resistance, needs to be performed. Rapid susceptibility tests are needed, since their results could play a direct role in the therapeutic decision.
References
Biron, K.K. (1991), Ganciclovir-resistant human cytomegalovirus clinical isolates; resistance mechanisms and in vitro susceptibility to antiviral agents. Transplant. Proc., 23 (suppl. 3), 162-167. Biron, K.K., Fyfe, J.A., Stanat, S.C. et al. (1986), A human cytomegalovirus mutant resistant to the nucleoside analog 9-[[2-hydroxy-l-(hydroxymethyl)ethoxy]methyl]-guanine (BW B759U) induces
reduced levels of BW B759U triphosphate. Proc. nat. Acad. Sci. (Wash.), 83, 8769-73. Cheng, Y.C., Huang, E.S., Lin, J.C. et al. (1983), Unique spectrum of activity of 9-[(l,3-dihydroxy-2-propoxy)methyl]-guanine against herpes viruses in vitro and its mode of action against herpes simplex virus type 1. Proc. nat. Acad. Sci. (Wash.), 80, 2767-2770. Cole, N.L. & Balfour, H.H. Jr. (1987), In vitro susceptibility of cytomegalovirus isolates from immunocompromised patients to acyclovir and ganciclovir. Diagn. Microbiol. infect. D/s., 6, 255-261. Dankner, W.M., Scholl, D., Stanat, S.C. et al. (1990), Rapid antiviral DNA-DNA hybridization assay for human cytomegalovirus. J. Virol. Methods, 28, 293 -298. Drew, W.L., Miner, R. & King, D. (1991), Antiviral activity of FIAU (1-[2'-deoxy-2'-fluoro-l-[3-d-arabinofuranosyl-]-5-iodo-uridine) on strains of cytomegalovirus sensitive and resistant to ganciclovir. J. infect. Dis., 163, 1389-1390. Drew, W.L., Miner, R.C., Bush, D.F. et al. (1991), Prevalence of resistance in patients receiving ganciclovir for serious cytomegalovirus infection. J. infect. Dis., 163, 716-719. Erice, A., Chou, S., Biron, K.K. et al. (1989), Progressive disease due to ganciclovir-resistant cytomegalovirus in immunocompromised patients. New Engl. J. Med., 320, 289-293. Field, A.K., Davies, M.E., De Witt, C. et al. (1983), 9-([2-hydroxy- l-(hydroxymethyl)ethoxy]methyl)-guanine: a selective inhibitor of herpes group virus replication. Proc. nat. Acad. Sci. (Wash.), 80, 4139-4143. Freitas, V.R., Smee, D.F., Chernow, M. et al. (1985), Activity of 9-(l,3-dihydroxy-2-propoxymethyl)guanine compared with that of acyclovir against human, monkey and rodent cytomegaloviruses. Antimicrob. Agents a. Chemother., 28, 240-245. Knox, K.K., Drobyski, W.R. & Carrigan, D.R. (1991), Cytomegaiovirus isolate resistant to ganciclovir and foscarnet from a marrow transplant patient. Lancet, II, 1292-1293. Mar, E.C., Cheng, Y.T., Huang, E.S. et al. (1983), Effect of 9-(l,3-dihydroxy-2-propoxymethyl)guanine on human cytomegalovirus replication in vitro. Antimicrob. Agents a. Chemother., 24, 518-521. Mazeron, M.C., St-Jean, L.A., Defer, M.C., Gluckman, E., Nebout, T. & P6rol, Y. (1989), Rapid method for determining susceptibility of human cytomegalovirus strains to antiviral drugs. IInd International Cytomegalovirus Worshop, San Diego, USA. Plotkin, S.A., Drew, W.L., Felsenstein, D. et al. (1985), Sensitivity of clinical isolates of human cytomegalovirus to 9-(l,3-dihydroxy-2-propoxymethyl)-guanine. J. infect. Dis., I52, 833-834. Rasmussen, L., Chen, P.T., Mullenax, J.G. et al. (1984), Inhibition of human cytomegalovirus replication by 9-(1,3-dihydroxy-2-propoxymethyl)-guanine alone and in combination with human interferons. Antimicrob. Agents a. Chemother., 26, 441-445. Sullivan, V. & Coen, D.M. (1991), Isolation of foscarnetresistant human cytomegalovirus patterns of resistance and sensitivity to other antiviral drugs. J. infect. Dis., 164, 781-784.
