Brian L. Robbins

Metabolic pathways for activation of the antiviral agent 9-(2-phosphonylmethoxyethyl)adenine in human lymphoid cells.

9-(2-Phosphonylmethoxyethyl)adenine (PMEA), the acyclic phosphonate analog of adenine monophosphate, is a promising antiviral drug with activity against herpesviruses, Epstein-Barr virus, and retroviruses, including the human immunodeficiency virus. In order to be active, it must be converted to the diphosphate derivative, the putative inhibitor of viral DNA polymerases. The metabolic pathway responsible for activation of PMEA is unclear. The metabolism of PMEA was investigated in human T-lymphoid cells (CEMss) and a PMEA-resistant subline (CEMss(r-1)) with a partial deficiency in adenylate kinase activity. Experiments with [3H]PMEA showed that extracts of CEMss phosphorylated PMEA to its mono- and diphosphate in the presence of ATP as the phosphate donor. No other nucleotides or 5-phosphoribosyl pyrophosphate displayed appreciable activity as a phosphate donor. Subcellular fractionation experiments showed that CEMss cells contained two nucleotide kinase activities, one in mitochondria and one in the cytosol, which phosphorylated PMEA. The PMEA-resistant CEMss mutant proved to have a deficiency in the mitochondrial adenylate kinase activity, indicating that this enzyme was important in the phosphorylation of PMEA. Other effective antiviral purine phosphonate derivatives of PMEA showed a profile of phosphorylating activity similar to that of PMEA. By comparison, phosphorylation of the pyrimidine analog (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl) cytosine proceeded by an enzyme present in the cytosol. We conclude from these studies that adenylate kinase which has been localized in the intermembrane space of mitochondria is the major route for PMEA phosphorylation in CEMss cells but that another hitherto unidentified enzyme(s) present in the cytosol may contribute to the anabolism of the phosphonates.

Mechanism of uptake of the phosphonate analog (S)-1-(3-hydroxy-2-phosphonylmethoxy-propyl)cytosine (HPMPC) in vero cells

The cellular uptake of phosphonylmethoxypropyl cytosine (HPMPC) was characterized to gain insight into the molecular properties that allow this anticytomegalovirus drug to permeate cell membranes. The time course of uptake of HPMPC into Vero cells was linear between 10 and 75 min and proportional to the concentration in the medium from 10(-6) to 10(-2) M. HPMPC uptake was temperature sensitive and the rate of uptake was considerably lower at 27 degrees than at 37 degrees and almost totally inhibited at 4 degrees. In competition studies with naturally occurring nucleosides, nucleotides or the phosphonylmethoxyethyl derivatives, none affected the uptake of HPMPC at concentrations up to 2000-fold molar excess. The uptake of [3H]HPMPC into Vero cells was compared with that of [14C]sucrose, a probe for fluid-phase endocytosis. Kinetics for both compounds were very similar, as were the effects of the microtubule antagonist colchicine and the tumor promoting agent phorbol myristate acetate. Colchicine and the phorbol ester are known to, respectively, inhibit and stimulate endocytosis. It is concluded from these data that HPMPC enters Vero cells by fluid-phase endocytosis and that once internalized it may accumulate in the lysosome. Protonation of the negative charge on the phosphonyl group in HPMPC may allow its diffusion across the lysosome membrane and eventual activation to its putative active diphosphorylated form in the cell cytoplasm.

Pharmacokinetics, safety and efficacy of lopinavir/ritonavir in infants less than 6 months of age: 24 week results.

Objective:To investigate pharmacokinetics, safety and efficacy of lopinavir/ritonavir (LPV/r)-based therapy in HIV-1-infected infants 6 weeks to 6 months of age. Methods:A prospective, multicenter, open-label trial of 21 infants with HIV-1 RNA > 10 000 copies/ml and treated with LPV/r 300/75 mg/m2 twice daily plus two nucleoside reverse transcriptase inhibitors. Intensive pharmacokinetic sampling was performed at 2 weeks and predose concentrations were collected every 8 weeks; safety and plasma HIV-1 RNA were monitored every 4–12 weeks for 24 weeks. Results:Median age at enrollment was 14.7 weeks (range, 6.9–25.7) and 19/21 completed ≥ 24 weeks of study. Although LPV/r apparent clearance was slightly higher than in older children, the median area under the concentration–time curve 0–12 h (67.5 μg.h/ml) was in the range reported from older children taking the recommended dose of 230/57.5 mg/m2. Predose concentrations stabilized at a higher level after the first 2 weeks of study. In as-treated analysis at week 24, 10/19 (53%) had plasma HIV-1 RNA < 400 copies/ml (median change, −3.33 log10 copies/ml); poor adherence contributed to delayed viral suppression, which improved with longer follow-up. Three infants (14%) had transient adverse events of grade 3 or more that were possibly related to study treatment but did not require permanent treatment discontinuation. Conclusion:Despite higher clearance in infants 6 weeks to 6 months of age, a twice daily dose of 300/75 mg/m2 LPV/r provided similar exposure to that in older children, was well tolerated and provided favorable virological and clinical efficacy.

Enzymatic assay for measurement of zidovudine triphosphate in peripheral blood mononuclear cells.

In this report, we describe a new method to measure intracellular zidovudine triphosphate (ZDV-TP) levels in peripheral blood mononuclear cells (PBMCs) from patients treated with ZDV by utilizing inhibition of human immunodeficiency virus type 1 reverse transcriptase activity by ZDV-TP. Intracellular levels of ZDV-TP were determined with our enzymatic assay in PBMCs isolated from the blood of healthy individuals incubated with different concentrations of labeled ZDV and were validated by high-performance liquid chromatography separation and liquid scintillation counting of the radioactive ZDV-TP. These methods gave virtually identical results over a range of ZDV-TP concentrations from 150 to 900 fmol. ZDV-TP recoveries were over 90%, and the limit of quantitation of ZDV-TP by this method was 20 to 50 fmol. To demonstrate the utility of the method, plasma ZDV and intracellular ZDV-TP concentrations were measured at serial time points over 6 h in 12 human immunodeficiency virus-infected volunteers following a single 100- or 500-mg oral dose of ZDV. Systemic oral clearance rates were similar to those in previous studies with adults but were highly variable (range, 0.86 to 2.75 liters/h/kg of body weight). The area under the plasma concentration versus time curve increased significantly (P < 0.0005) with the dose from a median value of 1.2 mg.h/liter at the lower dose to 4.2 mg.h/liter at the higher dose. Median intracellular ZDV-TP levels ranged from 5 to 57 and 42 to 92 fmol/10(6) cells in volunteers administered 100 and 500 mg of ZDV, respectively. Intracellular ZDV-TP levels rose to a plateau value by 2 h and remained consistent to 6 h. Although the higher dose and higher areas under the curve yielded consistently higher intracellular ZDV-TP levels, systemic pharmacokinetics explains only a modest proportion of the variability in cellular pharmacokinetic. The ZDV-TP bioassay should prove useful in further studies of ZDV metabolism in patient-derived PBMCs at the doses of ZDV currently administered.

Systemic Pharmacokinetics and Cellular Pharmacology of Zidovudine in Human Immunodeficiency Virus Type 1—Infected Women and Newborn Infants

Systemic and intracellular pharmacokinetics of zidovudine were determined for 28 human immunodeficiency virus type 1-infected pregnant women and their newborn infants. Plasma zidovudine and intracellular zidovudine monophosphate and triphosphate concentrations were determined in serial maternal samples and cord blood at delivery. Higher levels of cord blood zidovudine were associated with lower maternal zidovudine clearance and longer infusion times. Median levels of zidovudine monophosphate and triphosphate in maternal (1556 and 67 fmol/106 cells) and cord (1464 and 70 fmol/106 cells) blood were similar but highly variable. Intersubject pharmacokinetic variability for zidovudine is substantial, but intravenous therapy provides plasma concentrations and intracellular zidovudine triphosphate levels consistent with high antiviral activity. The substantial amount of intracellular zidovudine triphosphate in cord blood provides an explanation for the clinical success of zidovudine in reducing vertical transmission. Studies of simpler oral regimens of zidovudine can now be evaluated regarding the ability to achieve these pharmacologic end points associated with highly effective parenteral therapy.

Metabolism of tenofovir and didanosine in quiescent or stimulated human peripheral blood mononuclear cells

Objective. As tenofovir disoproxil fumarate substantially increases plasma concentrations of didanosine in patients with human immunodeficiency virus‐1 infection, we sought to determine whether tenofovir and didanosine showed a similar intracellular interaction in human peripheral blood mononuclear cells (PBMCs).

Early Initiation of Lopinavir/Ritonavir in Infants Less Than 6 Weeks of Age: Pharmacokinetics and 24 Week Safety and Efficacy

Background: With increasing recognition of the benefits of early antiretroviral therapy initiation in perinatally HIV-infected infants, data are needed regarding the pharmacokinetics (PK), safety, and efficacy of recommended first-line protease inhibitors such as lopinavir/ritonavir (LPV/r). Methods: A prospective, phase I/II, open-label, dose-finding trial evaluated LPV/r at a dose of 300/75 mg/m2 twice daily plus 2 nucleoside analogs in HIV-1-infected infants ≥14 days to <6 weeks of age. Intensive 12-hour PK evaluations were performed after 2 weeks of LPV/r therapy, and doses were modified to maintain LPV predose concentrations >1 μg/mL and area under the curve (AUC) <170 μg hr/mL. Results: Ten infants enrolled [median age 5.7 (range, 3.6–5.9) weeks] with median HIV-1 RNA of 6.0 (range, 4.7–7.2) log10 copies/mL; all completed 24 weeks of follow-up. Nine completed the intensive PK evaluation at a median LPV dose of 267 (range, 246–305) mg/m2 q12 hours; median measures were AUC = 36.6 (range, 27.9–62.6) μg hr/mL; predose concentration = 2.2 (range, 0.99–4.9) μg/mL; maximum concentration = 4.76 (range, 2.84–7.28) μg/mL and apparent clearance (L/h/m2) = 6.75 (range, 2.79–12.83). Adverse events were limited to transient grade 3 neutropenia in 3 subjects. By week 24, 2 of 10 subjects had experienced a protocol-defined virologic failure. Conclusions: Although the LPV AUC in this population was significantly lower than that observed in infants ages 6 weeks to 6 months, LPV/r-based antiretroviral therapy in doses of 300/75 mg/m2 BID was well tolerated and resulted in virologic control in 8 of 10 infants by 24 weeks. Additional investigation is needed to understand the long-term implications of the lower LPV exposure in this age group.

Pharmacokinetics and Safety of Single-Dose Tenofovir Disoproxil Fumarate and Emtricitabine in HIV-1-Infected Pregnant Women and Their Infants

ABSTRACT Tenofovir (TFV) is effective in preventing simian immunodeficiency virus (SIV) transmission in a macaque model, is available as the oral agent tenofovir disoproxil fumarate (TDF), and may be useful in the prevention of mother-to-child transmission of human immunodeficiency virus (HIV). We conducted a trial of TDF and TDF-emtricitabine (FTC) in HIV-infected pregnant women and their infants. Women received a single dose of either 600 mg TDF, 900 mg TDF, or 900 mg TDF-600 mg FTC at labor onset or prior to a cesarean section. Infants received no drug or a single dose of TDF at 4 mg/kg of body weight or of TDF at 4 mg/kg plus FTC at 3 mg/kg as soon as possible after birth. All regimens were safe and well tolerated. Maternal areas under the serum concentration-time curve (AUC) and concentrations at the end of sampling after 24 h (C24) were similar between the two doses of TDF; the maximum concentrations of the drugs in serum (Cmax) and cord blood concentrations were higher in women delivering via cesarean section than in those who delivered vaginally (P = 0.04 and 0.046, respectively). The median ratio of the TFV concentration in cord blood to that in the maternal plasma at delivery was 0.73 (range, 0.26 to 1.95). Without TDF administration, infants had a median TFV concentration of 12 ng/ml 12 h after birth. Following administration of a single dose of TDF at 4 mg/kg, infant TFV concentrations fell below the targeted level, 50 ng/ml, by 24 h postdose. In HIV-infected pregnant women and their infants, 600 mg of TDF is acceptable as a single dose during labor. Low concentrations at birth support infant dosing as soon after birth as possible. Rapidly decreasing TFV levels in infants suggest that multiple or higher doses of TDF will be necessary to maintain concentrations that are effective for viral suppression.

Long-term outcomes for HIV-infected infants less than 6 months of age at initiation of lopinavir/ritonavir combination antiretroviral therapy

Objective:To investigate the longitudinal pharmacokinetics, safety and efficacy of lopinavir/ritonavir (LPV/r) in HIV-infected infants initiating combination antiretroviral therapy (cART) between 2 weeks and 6 months of age. Method:A prospective, open-label, multicenter Phase I/II study of LPV/r-based cART at a dose of 300/75 mg/m2/dose LPV/r twice daily. Intensive pharmacokinetic sampling at 12 months of age and quarterly predose LPV concentrations were collected and safety, virologic and immunologic responses were monitored every 4–12 weeks up to 252 weeks. Results:Thirty-one HIV-infected infants enrolled into two age cohorts, 14 days to <6 weeks and 6 weeks to <6 months; 29 completed ≥48 weeks of follow-up (median = 123 weeks, range 4–252). At 12 months of age, median LPV area under the curve was comparable for both age cohorts and similar to older children and adults. At week 48, 22 of 31 patients (71%) had HIV-1 RNA <400 copies/ml and 11 of 15 (73%) had <50 copies/ml; 29 of 31 achieved HIV-1 RNA <400 copies/ml on study treatment and 19 (66%) remained durably suppressed until the end of study; viral suppression correlated with a higher percentage of predose time points exceeding the LPV target of 1 μg/ml (92 vs. 71%, P = 0.002). Conclusion:LPV/r at 300/75 mg/m2/dose as part of a cART regimen resulted in viral suppression through 96 weeks of treatment in >65% of young infants. Due to initially low LPV exposure in infants <6 weeks of age, frequent dose adjustment for weight gain is advisable and consideration should be given to studying a higher dose for very young infants.

Pharmacokinetics of High-Dose Lopinavir-Ritonavir with and without Saquinavir or Nonnucleoside Reverse Transcriptase Inhibitors in Human Immunodeficiency Virus-Infected Pediatric and Adolescent Patients Previously Treated with Protease Inhibitors

ABSTRACT Human immunodeficiency virus (HIV)-infected children and adolescents who are failing antiretrovirals may have a better virologic response when drug exposures are increased, using higher protease inhibitor doses or ritonavir boosting. We studied the pharmacokinetics and safety of high-dose lopinavir-ritonavir (LPV/r) in treatment-experienced patients, using an LPV/r dose of 400/100 mg/m2 orally every 12 h (p.o. q12h) (without nonnucleoside reverse transcriptase inhibitor [NNRTI]), or 480/120 mg/m2 p.o. q12h (with NNRTI). We calculated the LPV inhibitory quotient (IQ), and when the IQ was <15, saquinavir (SQV) 750 mg/m2 p.o. q12h was added to the regimen. We studied 26 HIV-infected patients. The median age was 15 years (range, 7 to 17), with 11.5 prior antiretroviral medications, 197 CD4 cells/ml, viral load of 75,577 copies/ml, and a 133-fold change in LPV resistance. By treatment week 2, 14 patients had a viral-load decrease of >0.75 log10, with a median maximal decrease in viral load of −1.57 log10 copies/ml at week 8. At week 2, 19 subjects showed a median LPV area under the concentration-time curve (AUC) of 157.2 (range, 62.8 to 305.5) μg·h/ml and median LPV trough concentration (Ctrough) of 10.8 (range, 4.1 to 25.3) μg/ml. In 16 subjects with SQV added, the SQV median AUC was 33.7 (range, 4.4 to 76.5) μg·h/ml and the median SQV Ctrough was 2.1 (range, 0.2 to 4.1) μg/ml. At week 24, 18 of 26 (69%) subjects remained in the study. Between weeks 24 and 48, one subject withdrew for nonadherence and nine withdrew for persistently high virus load. In antiretroviral-experienced children and adolescents with HIV, high doses of LPV/r with or without SQV offer safe options for salvage therapy, but the modest virologic response and the challenge of adherence to a regimen with a high pill burden may limit the usefulness of this approach.