12.4 Microbiology

Mechanism of Action: Valganciclovir is an L-valyl ester (prodrug) of ganciclovir that exists as a mixture of two diastereomers. After oral administration, both diastereomers are rapidly converted to ganciclovir by intestinal and hepatic esterases. Ganciclovir is a synthetic analogue of 2′-deoxyguanosine, which inhibits replication of human CMV in cell culture and in vivo.

In CMV-infected cells, ganciclovir is initially phosphorylated to ganciclovir monophosphate by the viral protein kinase, pUL97. Further phosphorylation occurs by cellular kinases to produce ganciclovir triphosphate, which is then slowly metabolized intracellularly (half-life 18 hours). As the phosphorylation is largely dependent on the viral kinase, phosphorylation of ganciclovir occurs preferentially in virus-infected cells. The virustatic activity of ganciclovir is due to inhibition of the viral DNA polymerase, pUL54 by ganciclovir triphosphate.

Antiviral Activity: The quantitative relationship between the cell culture susceptibility of human herpes viruses to antivirals and clinical response to antiviral therapy has not been established, and virus sensitivity testing has not been standardized. Sensitivity test results, expressed as the concentration of drug required to inhibit the growth of virus in cell culture by 50% (EC50 ), vary greatly depending upon a number of factors including the assay used. Thus, the reported EC50 values of ganciclovir that inhibit human CMV replication in cell culture (laboratory and clinical isolates) have ranged from 0.08 to 22.94 µM (0.02 to 5.75 mcg/mL). The distribution and range in susceptibility observed in one assay evaluating 130 clinical isolates was 0 to 1 µM (35%), 1.1 to 2 µM (20%), 2.1 to 3 µM (27%), 3.1 to 4 µM (13%), 4.1 to 5 µM (5%), less than 5 µM (less than 1%). Ganciclovir inhibits mammalian cell proliferation (CC50 ) in cell culture at higher concentrations ranging from 40 to greater than 1,000 µM (10.21 to greater than 250 mcg/mL). Bone marrow-derived colony-forming cells are more sensitive [CC50 value = 2.7 to 12 µM (0.69 to 3.06 mcg/mL)].
Viral Resistance:

Cell culture: CMV isolates with reduced susceptibility to ganciclovir have been selected in cell culture. Growth of CMV strains in the presence of ganciclovir resulted in the selection of amino acid substitutions in the viral protein kinase pUL97 (M460I/V, L595S, G598D, and K599T) and the viral DNA polymerase pUL54 (D301N, N410K, F412V, P488R, L516R, C539R, L545S, F595I, V812L, P829S, L862F, D879G, and V946L).

In vivo: Viruses resistant to ganciclovir can arise after prolonged treatment or prophylaxis with valganciclovir by selection of substitutions in pUL97 and/or pUL54. Limited clinical data are available on the development of clinical resistance to ganciclovir and many pathways to resistance likely exist. In clinical isolates, seven canonical pUL97 substitutions, (M460V/I, H520Q, C592G, A594V, L595S, and C603W) are the most frequently reported ganciclovir resistance-associated substitutions. These and other substitutions less frequently reported in the literature, or observed in clinical trials, are listed in Table 17.

Table 17 Summary of Resistance-associated Amino Acid Substitutions Observed in the CMV of Patients Failing Ganciclovir Treatment or Prophylaxis
Note: Many additional pathways to ganciclovir resistance likely exist
pUL97 F342Y, K359E/Q,L405P, A440V, M460I/V/T/L, V466G/M, C480F, C518Y, H520Q, P521L, del 590-593, A591D/V, C592F/G, A594E/G/T/V/P, L595F/S/T/W, del 595, del 595-603, E596D/G/Y, K599E/M, del 600-601, del 597­ 600, del 601-603, C603W/R/S/Y, C607F/S/Y, I610T, A613V
pUL54 E315D, N408D/K/S, F412C/L/S, D413A/E/N, L501F/I, T503I, K513E/N/R, D515E, L516W, I521T, P522A/L/S, V526L, C539G, L545S/W, Q578H/L, D588E/N, G629S, S695T, I726T/V, E756K, L773V, V781I, V787E/L, L802M, A809V, T813S, T821I, A834P, G841A/S, D879G, A972V, del 981­ 982, A987G

The presence of known ganciclovir resistance-associated amino acid substitutions was evaluated in a study that extended valganciclovir CMV prophylaxis from 100 days to 200 days post-transplant in adult kidney transplant patients at high risk for CMV disease (D+/R-) [see Clinical Studies (14.1)]. Five subjects from the 100 day group and four subjects from the 200 day group meeting the resistance analysis criteria had known ganciclovir resistance-associated amino acid substitutions detected. In six subjects, the following resistance-associated amino acid substitutions were detected within pUL97: 100 day group: A440V, M460V, C592G; 200 day group: M460V, C603W. In three subjects, the following resistance-associated amino acid substitutions were detected within pUL54: 100 day group: E315D; 200 day group: E315D, P522S. Overall, the detection of known ganciclovir resistance-associated amino acid substitutions was observed more frequently in patients during prophylaxis therapy than after the completion of prophylaxis therapy (during therapy: 5/12 [42%] versus after therapy: 4/58 [7%]). The possibility of viral resistance should be considered in patients who show poor clinical response or experience persistent viral excretion during therapy.

Cross-Resistance: Cross-resistance has been reported for amino acid substitutions selected in cell culture by ganciclovir, cidofovir or foscarnet. In general, amino acid substitutions in pUL54 conferring cross-resistance to ganciclovir and cidofovir are located within the exonuclease domains and region V of the viral DNA polymerase. Whereas, amino acid substitutions conferring cross-resistance to foscarnet are diverse, but concentrate at and between regions II (codon 696 to 742) and III (codon 805 to 845). The amino acid substitutions that resulted in reduced susceptibility to ganciclovir and either cidofovir and/or foscarnet are summarized in Table 18.

Substitutions at amino acid positions pUL97 340 to 400 have been found to confer resistance to ganciclovir. Resistance data based on assays that do not include this region should be interpreted cautiously.

Table 18 Summary of pUL54 Amino Acid Substitutions with Cross-Resistance between Ganciclovir, Cidofovir, and/or Foscarnet
Cross-resistant to cidofovir D301N, N408D/K, N410K, F412C/L/S/V, D413E/N, P488R, L501I, T503I, K513E/N, L516R/W, I521T, P522S/A, V526L, C539G/R, L545S/W, Q578H, D588N, I726T/V, E756K, L733V, V787E, V812L, T813S, A834P, G841A, del 981-982, A987G
Cross-resistant to foscarnet F412C, Q578H/L, D588N, V715A/M, E756K, L733V, V776M,V781I, V787E/L, L802M, A809V, V812L, T813S, T821I, A834P, G841A/S, del 981-982


13.1 Carcinogenesis & Mutagenesis & Impairment Of Fertility

Long-term carcinogenicity studies have not been conducted with valganciclovir. However, upon oral administration, valganciclovir is rapidly and extensively converted to ganciclovir. Therefore, like ganciclovir, valganciclovir is a potential carcinogen.

Ganciclovir was carcinogenic in the mouse at oral doses that produced exposures approximately 0.1x and 1.4x, respectively, the mean drug exposure in humans following the recommended intravenous dose of 5 mg/kg, based on area under the plasma concentration curve (AUC) comparisons. At the higher dose, there was a significant increase in the incidence of tumors of the preputial gland in males, forestomach (nonglandular mucosa) in males and females, and reproductive tissues (ovaries, uterus, mammary gland, clitoral gland and vagina) and liver in females. At the lower dose, a slightly increased incidence of tumors was noted in the preputial and harderian glands in males, forestomach in males and females, and liver in females. Ganciclovir should be considered a potential carcinogen in humans.

Valganciclovir increases mutations in mouse lymphoma cells. In the mouse micronucleus assay, valganciclovir was clastogenic. Valganciclovir was not mutagenic in the Ames Salmonella assay. Ganciclovir increased mutations in mouse lymphoma cells and DNA damage in human lymphocytes in vitro. In the mouse micronucleus assay, ganciclovir was clastogenic. Ganciclovir was not mutagenic in the Ames Salmonella assay.

Valganciclovir is converted to ganciclovir and therefore is expected to have similar reproductive toxicity effects as ganciclovir [see Warnings and Precautions (5.3)]. Ganciclovir caused decreased mating behavior, decreased fertility, and an increased incidence of embryolethality in female mice following intravenous doses that produced an exposure approximately 1.7x the mean drug exposure in humans following the dose of 5 mg per kg, based on AUC comparisons. Ganciclovir caused decreased fertility in male mice and hypospermatogenesis in mice and dogs following daily oral or intravenous administration. Systemic drug exposure (AUC) at the lowest dose showing toxicity in each species ranged from 0.03 to 0.1x the AUC of the recommended human intravenous dose. Valganciclovir caused similar effects on spermatogenesis in mice, rats, and dogs. These effects were reversible at lower doses but irreversible at higher doses. It is considered likely that ganciclovir (and valganciclovir) could cause temporary or permanent inhibition of human spermatogenesis.

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