Authors: Gheshlaghi N et al.
Anesthesia & Analgesia 142(2):280-283, February 2026.
This article discusses how pharmacokinetic model-based simulations may help guide opioid dosing decisions in pediatric patients, particularly when genetic differences influence drug metabolism and transport. Despite advances in pediatric analgesia, pain control remains inadequate for many children, with approximately one in four hospitalized pediatric patients receiving little or no relief from current treatments. Morphine remains a commonly used opioid for moderate-to-severe pain in children, but individual responses vary widely due to pharmacogenetic differences.
A key focus of this article is the hepatic transporter gene SLC22A1, which encodes the organic cation transporter OCT1. This transporter plays an important role in hepatic uptake and clearance of morphine. More than 20 genetic variants of SLC22A1 have been identified, and several loss-of-function alleles significantly reduce hepatic transport activity. Individuals who carry two of these loss-of-function variants may have reduced hepatic morphine uptake, resulting in higher systemic drug exposure.
The authors used a previously published pediatric pharmacokinetic model and adapted it to simulate both intravenous and oral morphine administration in a typical six-year-old child. The simulations compared morphine plasma concentrations in children with two SLC22A1 loss-of-function alleles versus those with normal transporter function after a standard dose of 0.2 mg/kg.
The simulations predicted meaningful differences in morphine exposure depending on both genotype and route of administration. After intravenous dosing, children with two loss-of-function alleles demonstrated approximately 20% higher morphine exposure compared with other genotypes. However, the effect was substantially larger with oral administration, where exposure increased by approximately 52%.
Peak plasma concentrations were also influenced by genotype. After oral morphine administration, maximum concentrations were predicted to be about 45% higher in children with two loss-of-function alleles. In contrast, no meaningful difference in peak concentration was observed after intravenous dosing.
These findings reflect morphine’s pharmacokinetic characteristics. Morphine is a high hepatic extraction drug. When administered intravenously, the drug bypasses first-pass hepatic metabolism, so genetic differences in hepatic transport have a smaller effect. With oral administration, however, morphine undergoes significant first-pass hepatic extraction. In this setting, reduced transporter function increases both oral bioavailability and systemic exposure.
Based on these model predictions, the authors estimate that achieving similar morphine exposure across genotypes might require approximately a 17% dose reduction for intravenous morphine and a 34% dose reduction for oral morphine in children carrying two SLC22A1 loss-of-function alleles.
The article also highlights other contributors to variability in morphine pharmacokinetics. Genetic differences in metabolic enzymes such as UGT2B7 may alter morphine glucuronidation and further affect exposure. Drug interactions may also influence transporter function. For example, medications such as ondansetron, irinotecan, and verapamil may inhibit SLC22A1 activity, potentially increasing morphine concentrations. Developmental factors, hepatic function, renal function, and comorbid conditions also contribute to variability in pediatric opioid responses.
The authors emphasize that pharmacokinetic simulations offer a valuable research tool in pediatric populations, where large pharmacogenetic trials are often difficult due to ethical and logistical challenges. By integrating existing pharmacokinetic data with genetic information, simulations can help generate dosing hypotheses while minimizing the need for additional pediatric trials.
The increasing availability of rapid pharmacogenetic testing—using small blood samples or noninvasive buccal swabs—may further enable personalized opioid dosing in the future. Incorporating pharmacogenetic data with pharmacokinetic modeling could help clinicians better predict morphine exposure and optimize analgesic therapy for individual pediatric patients.
Key Points
• Pediatric pain control remains inadequate, with many children receiving insufficient analgesia.
• Genetic variation in the hepatic transporter SLC22A1 significantly influences morphine pharmacokinetics.
• Pharmacokinetic simulations predicted a 20% increase in morphine exposure after IV dosing in children with two loss-of-function alleles.
• Oral morphine exposure increased by approximately 52% in the same genotype group due to first-pass hepatic effects.
• Model predictions suggest potential dose reductions of about 17% for IV morphine and 34% for oral morphine in affected patients.
• Pharmacokinetic simulations may help guide personalized opioid dosing in pediatric populations.
Thank you to Anesthesia & Analgesia for allowing us to summarize and share this article.