Pharmacology of protoporphyrin IX in nude mice after application of ALA and ALA esters

Abstract

Aminolevulinic acid (ALA), ALA methylester (ALA-Me) and ALA hexylester (ALA-Hex) were topically applied for 5 and 20 hr, respectively, on normal skin of mice. The distribution of protoporphyrin IX (PpIX) induced in 7 different tissues by these drugs was determined either by spectrofluorometric measurements with an optical fibre probe or by chemical extraction of PpIX from the tissues. The results from these 2 types of measurements were compared. Both methods showed that ALA and the esters induced similar amounts of PpIX at the skin spot where they were applied and that the esters produced much less PpIX at remote skin spots (i.e., spots outside the location where the drugs were applied) than ALA did, notably after 20 hr application. After 20 hr of drug application ALA produced much more PpIX in liver, intestine and lungs than the esters did. In contrast with the direct fluorescence measurements, the extraction method showed detectable amounts of PpIX in liver, intestine and lung after application of the esters, notably of ALAMe. The discrepancy is probably related to the fact that the pigmented tissues absorb light and, therefore, the direct fluorescence readings are misleading. Notably in the liver, which contains high concentration of light-absorbing pigments, very weak direct fluorescence was seen. In no case there was any accumulation of PpIX in muscle tissue nor in brain. The esters seem to penetrate less into the circulation than ALA, and PpIX formed by them in the skin is faster cleared than PpIX formed from ALA. This is also true after oral and i.p. administration of the drugs. © 2002 Wiley-Liss, Inc. Key words: ALA; ALA-esters; pharmacology; PDT; protoporphyrin IXThe use of 5-aminolevulinic acid (ALA) for fluorescence detection (FD) and photodynamic therapy (PDT) of tumors has several advantages over traditional PDT, where photosensitizing dyes are injected intravenously or given orally. First, ALA can be applied topically because ALA molecules are small enough to penetrate the stratum corneum. 1 Second, ALA-induced protoporphyrin IX (PpIX) is cleared from the body more rapidly than other sensitizers, and, therefore, gives less and shorter lasting cutaneous photosensitization.A limitation of ALA-PDT is the shallow penetration of ALA into tissues. ALA-esters, which are more lipophilic than ALA, 2 were introduced to get a deeper penetration and thus facilitate the treatment of larger tumors. [3][4][5] Porphyrins in biological samples, such as cells and skin, are often quantified by direct fluorescence readings. It is well known, however, that these dyes, notably lipophilic ones like PpIX, have a strong tendency to aggregate in aqueous solutions. 6,7 Furthermore, the aggregates have much lower fluorescence quantum yields and photosensitizing efficiencies than monomers. 6 -8 Different degrees of aggregation may take place in different organs. This, in addition to different degrees of pigmentation, makes direct fluorescence measurements of limited value in pharmacological studies where concentration levels i...

Influence of Light Exposure on the Kinetics of Protoporphyrin IX Formation in Normal Skin of Hairless Mice After Application of 5-Aminolevulinic Acid Methyl Ester

“…The kinetics of PpIX formation induced by ALA-Me in mouse skin observed in this work (Fig 1) are in general agreement with earlier findings (Moan et al, 2003). The main purpose of this work was to study PpIX reappearance after light exposures that degrade a substantial fraction of the PpIX present in the skin.…”

Quantification of in vivo fluorescence decoupled from the effects of tissue optical properties using fiber-optic spectroscopy measurements

“…(9). Another potential source of error in the 21 used intraperitoneal injection at 1.5 mM/kg, and the results seem to be consistent (even though our data has only N = 1 per time point). It is encouraging that there is no apparent significant systematic bias introduced by the fiber optic probe technique, so that it should be reliable for time-dependent or longitudinal studies.…”

Increased brain tumor resection using fluorescence image guidance in a preclinical model

“…We have shown previously [17][18][19][20] that ALA-induced PpIX levels in normal brain tissue, especially white matter, are very low. Clinical and preclinical studies suggest that the resulting selectivity of ALA-PpIX in certain brain tumors [14,[17][18][19][20][21][22][23][24][25] is a result of various factors. The low permeability of ALA at the blood-brain barrier (BBB) [17,26] reduces uptake in normal brain, whereas the compromised BBB in certain brain tumors is thought to permit selective ALA transport.…”