Matrix metalloproteinases (MMPs) degrade extracellular proteins during epithelialization of wounds. To evaluate the biological significance of MMPs in epidermal healing, the synthetic broad-spectrum MMP inhibitor GM 6001 (also called Galardin and Ilomastat) was applied topically to standardized human wounds. GM 6001 (10 microg/microl) or vehicle alone was applied every second day onto 4 de-roofed 6 mm suction blister wounds on the volar forearm of healthy male volunteers for 12 days. GM 6001 delayed healing by 2-4 days as assessed macroscopically and microscopically. In situ hybridization or immunohistochemistry showed that MMP-1 (interstitial collagenase) was present in and MMP-2 (gelatinase A) close to laterally migrating keratinocytes whereas MMP-9 (gelatinase B) was seen during maturation of new epidermis. MMP-1 was undetectable in blister roofs (normal epidermis) and found in low levels in normal skin. Total MMP-1 activities increased about 100-fold in wounds, independent of treatment, compared to normal skin as analyzed by specific ELISA-based activity assay. By gelatin zymography, MMP-2, but not MMP-9, was detected in blister roofs and wound healing was associated with increased active MMP-2 and latent MMP-9 levels. GM 6001 prevented activation of MMP-2 and increased latent MMP-9 levels. GM 6001 delayed re-appearance of laminin-5, the synthesis of which correlated with epidermal regeneration. Restoration of stratum corneum, measured indirectly by transepidermal water loss, was also impaired (P<0.05) in the GM 6001 group. In conclusion, pharmacological MMP inhibition delayed epidermal regeneration in vivo, suggesting that MMPs are required to restore epidermis after epidermal ablation in humans.