Double blind cross-over studies on phototoxicity to three tetracycline derivatives in human volunteers.

A double blind cross-over phototoxicity study using demethylchlortetracycline (DMCT) 0.3 g x 2, doxycycline 0.1 g x 2, lymecycline 0.6 g x 2 and placebo was performed on 8 healthy human volunteers. Drugs were given for 3 consecutive days, and on the third day the volunteers were tested with different modalities of artificial long-wave ultraviolet radiation (UVA) and assessed 24 h later for objective as well as subjective abnormal photoreactions. All 4 substances were tested in each individual at weekly intervals and serum concentrations of tetracycline were determined. It was found most convenient to irradiate relatively large skin areas using fluorescent tubes emitting mainly UVA but also a small proportion of UVB. Using this type of irradiation, weak erythemal reactions were obtained with all 3 derivatives. Taking only stronger reactions and stinging sensations into account, 0/8 reacted to lymecycline, 0/8 to DMCT and 4/8 to doxycycline. There was no significant difference in serum concentration among the 3 derivatives. It is concluded that doxycycline is the most potent photosensitizer at the dosage tested.