Paediatric Antiretroviral PK University of Liverpool, UK David Back University of Liverpool, UK

Pediatric Developmental Pharmacology

PK in Paediatric Populations Developmental changes can significantly affect ADME Majority of PK data in paediatric patients obtained in older children Substantial intra and inter patient variability

Issues for dosing of ART in children Large variability in pharmacokinetic (PK) parameters age (and PK data by age-group often sparse) effect of nutritional status ethnicity Methods of dose calculation (per m2 or per kg) Ability to give with/without food (ddI, NFV) High within and between individual variability and PK Age a very important facros – high doses required in infants in particular for most drugs esp PI and NNRTIs Bioequivalence varies by formulationi – eg efv need to give higher dose of lquid

Impact of Nutrition on PK (Can have a profound effect) Diminished protein status in malnourished children results in lower plasma proteins Increasing concentrations of ‘free’ drug Severely malnourished children have decreased CYP450 metabolism Reduced hepatic clearance Severely malnourished children have decreased GFR Reduced renal clearance Murry et al Int J Cancer 1998; 11: 48-51 Jorquera F et al Nutrition 1996; 12: 442-447

Same dose but different plasma concentrations Mechanism of Genetic Variability in Drug Response Same dose but different plasma concentrations Drug A AUC 1 10 Concentration GCCCCGCCTC P 450 1 wild type Time Drug B AUC 20 10 GCCCCACCTC Concentration P450 1 mutation Time ME3012.PPT

A Common CYP2B6 Variant Associated with EFV PK and CNS Side Effects Slide #7 A Common CYP2B6 Variant Associated with EFV PK and CNS Side Effects A CYP2B6 polymorphism. More common in African-Americans than European-Americans. Associated with higher EFV levels, and increased CNS AE’s. Additional studies needed.

Left Hand Y Axis EFV concentration on a log scale Resistance WT at wk 6 Left Hand Y Axis EFV concentration on a log scale Orientation slide, blue points EFV conc Second right hand y axis plasma viral load log scale represented by green tiangles viral load over time Vertical arrows 1,2 and 3 weeks Lower Horizontal red lines protein corrected EC95 for EFV 92 Upper red line lower end of therapeutic range Horizontal bar drugs patient continued to take 0 patient stopped EFV S. Taylor et al. 11th CROI Abs 131

Finally T1/2 > 200 hours although LFTs not deranged This is the 3rd African woman with therapeutic levels at week 2 +/- week 3 Intersetingly she also developed toxicty with NVP data on nvp levels to be added Viral load and NVP data needs adding S. Taylor et al. 11th CROI Abs 131

Note: Difference between plasma and intracellular half life Time (hours) 5 10 15 20 25 1 100 1000 10000 Intracellular Carbovir-TP t1/2 20.64h Plasma Abacavir t1/2 2.59 h Intracellular CBV-TP, fmol/million cell Plasma Abacavir, ng/mL Piliero P, et al. 43rd ICAAC 2003, Abstr. A-1797

NRTI (intracellular) and NNRTI half lives ZDVTP 7 h NVP 25-30 h d4TTP 7 h EFV 35 h 3TCTP 16 h CBVTP 20 h ddATP 25 h TDFDP 60 h FTCTP 39 h

Balancing drugs with different half lives Last Dose Day 1 Day 2 MONOTHERAPY Drug concentration Key Points Considerations of forgiveness are critical for ensuring “coverage” if a dose is missed IC90 Zone of potential replication IC50 12 24 36 48 Time (hours) S. Taylor et al. 11th CROI Abs 131

3TC Clearance in Children Sokol E, et al. AAC 2000, 44:590-97

NVP Concentrations in Children by Age < 2 years 2-8 years > 8 years 2500 5000 7500 10000 12500 15000 17500 20000 22500 NVP Concentration (ng/ml)

Paediatric Nevirapine Concentrations (twice daily regimens) 3400 ng/ml 26.0% (20/77) below target

Paediatric Nevirapine Concentrations (twice daily regimens) 23.4% (18/77) above target 8000 ng/ml

Nelfinavir PK in Children Children <2 y at risk of subtherapeutic NFV levels Bergshoeff et al, 2002

Nelfinavir Troughs with TID and BID Dosing NFV PK were evaluated in 35 children (8.1 ± 3.5 yrs) receiving 20-30 mg/kg q8h or 50 mg/kg q12 h with food. Trough values were: 1.55 mg/L (0.13-5.22 mg/L) for TID dosing 1.11 mg/L (nd-6.08 mg/L) with BID. 1/11 (9%) in TID group vs. 7/14 (50%) in BID group had values < 1 mg/L (p=0.042). * Gatti G, et al. Clin Infect Dis, 2003;36:1476-82.

Nelfinavir Use in Children The proportion of children 2-13 years of age achieving an HIV RNA level < 400 cpm through 48 wks ranged from 26-42%. Response rates in children < 2 years of age appeared to be poorer than those ≥ 2 years. Highly variable exposure remains a significant problem in the use of nelfinavir in pediatric patients. Viracept Package Insert, March 29, 2004

LPV Concentrations in Children by Age 50000 40000 30000 LPV Concentration (ng/ml) 20000 10000 < 2 years 2-8 years > 8 years

Lopinavir/r (300/75 mg/m2 BID) Pharmacokinetics in Children All Subjects (N=27) No NVP (5 ≤ 2 yrs) With NVP (2 ≤ 2 yrs) Tmax (h) 4 ± 2.1 4 ± 2 4 ± 2.3 Cmax (mg/L) 11.4 ± 4.9 12.5 ± 5.8 10.0 ± 3.3 Cmin (mg/L) 5.2 ± 4.3 6.53 ± 4.6 3.6 ± 3.5 Cpre (mg/L) 6.9 ± 4.1 7.9 ± 4.5 5.6 ± 3.3 AUC12 (mg•h/L) 102.8 ± 50.7 116.4 ± 57.1 85.8 ± 36.9 T1/2 (h) 6.1 ± 5.2 7.6 ± 5.1 4.7 ± 4.5 X. Saez-Llorens et al. Ped Infect Dis J 2003;22:216-23.

Reduced Lopinavir Plasma Concentrations in Pregnancy 10 9 8 7 6 Median (± SE) Lopinavir (μg/mL) 5 4 3 2 Protein-adjusted IC50 for LPV 1 One of the more important issues that has emerged in pharmacology relates to the use of antiretroviral drugs in pregnancy. Previous data had suggested that pregnancy—particularly in the third trimester—might be associated with lower protease inhibitor exposures, although not necessarily to a clinically significant degree. The Pediatric ACTG examined boosted lopinavir in a cohort of patients between 30 and 36 weeks of pregnancy, and also examined the same patients 6 to 12 weeks after delivery. The study was designed to compare the drug exposures in these women to the expected levels derived from nonpregnant women. As shown in the slide, during pregnancy (shown in orange) the concentrations of lopinavir were markedly lower than one would expect in a nonpregnant woman. Postpartum, the lopinavir concentrations rose, but not to the levels that one would anticipate in nonpregnant women. The implications of these findings are unclear. The majority of women in this study maintained virologic suppression while receiving lopinavir/ritonavir with 2 NRTIs. However, the reduction in plasma concentrations is a concern, and has led the Pediatric ACTG to amend this study to examine higher doses of boosted lopinavir during the third trimester of pregnancy to see if one can consistently achieve acceptable drug levels. Another study at the meeting evaluated nevirapine and showed that concentrations of nevirapine were reduced in pregnancy compared with nonpregnant women, but in that study, the degree of reduction was unlikely to be clinically relevant. For more information, please go online to: http://clinicaloptions.com/hiv/conf/iac2004/cs/8.asp 1 2 3 4 5 6 7 8 9 10 11 12 13 Time Post Dose (hours) Pregnancy (n = 17) Postpartum (n = 8) Nonpregnant historical controls Note also abstract 4644 – NVP plasma exposure reduced in pregnant vs nonpregnant women Stek et al. Abstract LBOrB08.

Key Points Incomplete knowledge of pharmacology Marked Inter individual variability Nutritional status & PK Age group & PK Ethnicity & PK Dosing according to weight or BSA seems arbitary. Equivalence – Pharmaceutic & Bioequivalence Bd vs qd The future of PIs in children 1st line to ?????

Nelfinavir Pharmacokinetics in Children vs. Adults