Photosensitization of murine tumor, vasculature and skin by 5-aminolevulinic acid-induced porphyrin.
The effects of topical and systemic administration of 5-aminolevulinic acid (ALA) were examined in several murine tumor systems with regard to porphyrin accumulation kinetics in tumor, skin and blood, vascular and tumor cell photosensitization and tumor response after light exposure. Marked, transient increases in porphyrin levels were observed in tumor and skin after systemic and topical ALA. Rapid, transient, dose-dependent porphyrin increases were also observed in blood; these were pronounced after systemic ALA injection and mild after topical application. They were highest within 1 h after ALA injection, thereafter declining rapidly. This matched the clearing kinetics of injected exogenous protoporphyrin IX (PpIX). Initially, vascular photosensitivity changed inversely to blood porphyrin levels, increasing gradually up to 5 h post-ALA, as porphyrin was clearing from the bloodstream. This pattern was again matched by injected, exogenous PpIX. After therapeutic tumor treatment vascular disruption of the tumor bed, while observed, was incomplete, especially at the tumor base. Minimal direct tumor cell kill was found at low photodynamic therapy (PDT) doses (250 mg/kg ALA, 135 J/cm2 light). Significant, but limited (< 1 log) direct photodynamic tumor cell kill was obtained when the PDT dose was raised to 500 mg/kg systemic ALA, followed 3 h later by 270 J/cm2, a dose that was however toxic to the animals. The further reduction of clonogenic tumor cells over 24 h following treatment was moderate and probably limited by the incomplete disruption of the vasculature. Tumor responses were highest when light treatment was carried out at the time of highest tumor porphyrin content rather than at the time of highest vascular photosensitivity. Tumor destruction did not reach the tumor base, regardless of treatment conditions.