Percutaneous absorption of testosterone in the newborn rhesus monkey: comparison to the adult.

Percutaneous absorption of testosterone was determined in newborn rhesus monkeys, an animal model which is relevant to man. Mean percentage of absorptions of 4 and 40 microng/cm2 in the newborn were, respectively, 22.5 +/- 2.2 (SD) and 6.8 +/- 2.1. Statistical comparisons (Student's t-test) of these results with those obtained with adults show no significant difference (P greater than 0.05) in skin penetration of testosternoe in newborn and adult rhesus monkeys. In the newborn, the efficiency of absorption (percentage) decreased when the topical dose was increased 10-fold. However, the total compound absorbed per cm2 area of skin actually increased from 0.9 to 2.7 microng. With one other newborn rhesus, a topical dose of 40 microng/cm2 was applied to the ventral forearm and the area was occluded for 24 hr. Percutaneous absorption was 14.7%, a value twice that from nonoccluded absorption. Systemic absorption from a topical dose becomes critical in the newborn because the ratio of surface area (cm2) to body weight (kilograms) in the newborn is 3 times that in the adult. Given equal application area of skin per newborn and adult, the systemic absorption in the newborn becomes 3 times that of the adult when based on kilograms body weight. With a different ratio of skin surfact to body weight, the therapeutic ratio probably is lower in the newborn than in the adult when the compound is applied topically.