Oral treatment with retinoic acid decreases bone mass in rats

13-cis-retinoic acid (13-cis-RA, isotretinoin) is used to treat severe recalcitrant acne. Other retinoids have adverse effects on bone. Recent studies of human patients treated with 13-cis-RA have had varying results, perhaps because of variability among patients and the lack of control groups. The effects of retinoids have been studied in rodents, but little information is available regarding the effects of clinically relevant retinoid doses as evaluated by use of bone densitometric techniques. We treated rats for 15 or 20 wk with 13-cis-RA, all-trans-RA, or soybean oil (control) by gavage. We used dual-energy X-ray absorptiometry, histomorphometry, and histologic evaluation to evaluate effects on bone. Spontaneous long bone fractures occurred in some rats treated with 15 mg/kg all-trans-RA daily. Bone mineral density, bone mineral content, bone diameter, and cortical thickness of the femur were reduced in rats treated daily with 10 or 15 mg/kg all-trans-RA or 30 mg/kg 13-cis-RA. The lumbar spine was not affected. Although the effects of 13-cis-RA were not as dramatic as those of all-trans-RA, further study of the effects of 13-cis-RA on long bones is warranted.