IN-VIVO EVALUATION OF THE PENETRATION OF TOPICALLY APPLIED DRUGS INTOHUMAN SKIN BY SPECTROSCOPIC METHODS

Spectroscopic techniques are reported on which allow to study in vivo the penetration behaviour of topically applied light-absorbing drugs i nto human skin. Remittance specroscopy, a purely optical method, provi des a good tool in both, skin adaptation by use of a remote viewing he ad coupled to the spectrometer via optical fibres, and adequate sensit ivity for the detection of small amounts of the applied drugs. The mea suring depth in the skin is determined by the wavelength-dependent opt ical penetration depth, which itself depends on light absorption and l ight scattering, In the UV-spectral region the optical penetration dep th is of the order of the thickness of the stratum corneum (UV-A) or o f only a superficial part of it (UV-B, UV-C). Fluorescence spectroscop y, another optical method, offers two kinds of drug detection, a direc t one in case of self-fluorescent drugs or an indirect one being based on the light absorption of the drug, which may give rise to a screeni ng of the self-fluorescence of the skin itself or of an applied marker . The measuring depth is comparable to that achieved with remittance s pectroscopy. A third method is photothermal spectroscopy which is dete rmined by thermal properties of the skin in addition to optical proper ties. Photothermal spectroscopy is unique in that it allows depth prof iles of drug concentration to be measured non-invasively, as the photo thermal measuring depth can be changed by varying the modulation frequ ency of the intensity-modulated incident light. Results of measurement s demonstrating the potentials of these spectroscopic methods are pres ented.