Table 1. A brief overview of hepatic Phase I and II drug metabolizing enzymes ontogeny in Pediatric population*.

Pediatric Population	Duration	Enzymes in the Maturation Phase	Enzymes Already Matured	Enzymes with Higher Activity	Enzymes with Lower Activity	Absence of Enzyme Activity
Fetus & Preterm infants	<37 weeks gestation	CYP2C9, 19; CYP2E1 GSTA1 SULT1A1	CYP1A1 CYP2J2 SULT2A1 SULT1A3	CYP1A1 CYP2J2 GSTP1 SULT1E1 SULT1A3	CYP2D6¶	CYP1A2 CYP2A(all) CYP2B6 CYP3A4¶ CYP2C9
Term newborn infants or newborns	0-28 days	CYP2D6 CYP2C19 CYP3A4 CYP2C9 GSTA1 SULT1A1	CYP2D6 CYP2E1	SULT2A1 -	SULT1A3¶	CYP1A2
Infants & toddlers	>28 days to 23 months	CYP1A2 CYP2B6 CYP2C19 CYP3A4	CYP2A6 CYP2C9 CYP2E1	CYP2A6 CYP2E1 SULT2A1	CYP1A2 SULT1E1	CYP1A1
Children	2-11 years	CYP2C19 GSTM1	CYP2D6 CYP2A6 GSTA1 SULT1A1	CYP2D6 CYP2A6 SULT2A1	SULT1E1-	CYP1A1
Adolescents	12 to 16/18 years	CYP2C19	CYP2D6 CYP1A2 GSTM1 SULT2A1 SULT1A1	CYP2D6 CYP1A2 CYP2A6 CYP2C9 CYP2C19 SULT2A1	CYP2B6 SULT1E1	CYP1A1 SULT1A3

*Data presented here is ambiguous and concluded from studies which includes either measurement of mRNA expression [6] or Specific protein quantification [7] and probe drug metabolism in a limited number of samples [8]. ¶These enzymes are also absent or present at very low levels.