Selectivity of the photosensitiser Tookad® for photodynamic therapy evaluated in the Syrian golden hamster cheek pouch tumour model

The response to photodynamic therapy (PDT) with the photosensitiser (PS) Tookad was measured in the Syrian hamster cheek pouch model on normal mucosae and chemically induced squamous cell carcinoma. This PS is a palladium-bacteriopheophorbide presenting absorption peaks at 538 and 762 nm. The light dose, drug dose and drug injection-light irradiation times (DLI), ranging between 100 and 300 J cm(-2), 1-5 mg kg(-1) and 10-240 min respectively, were varied and the response to PDT was analysed by staging the macroscopic response and by the histological examination of the sections of the irradiated cheek pouch. A fast time decay of the tissular response with drug dose of 1-5 mg kg(-1) was observed for DLI ranging from 10 to 240 min and for light doses of 100-300 J cm(-2) delivered at a light dose rate of 150 mW cm(-2). A significantly higher level of tissular response was observed for squamous cell carcinoma compared to normal tissue. Nevertheless, the threshold level of the drug-light dose for a detectable response was not significantly different in the tumoral vs normal tissue. The highest response at the shortest DLIs and the absence of measurable response at DLI larger than 240 min at light dose of 300 J cm(-2) and drug dose of 5 mg kg(-1) reveals the predominantly vascular effect of Tookad. This observation suggests that Tookad could be effective in PDT of vascularised lesions

Cooperative polymerization of photosynthetic pigments in formamide-water solution

The aggregation of bacteriochlorophyll a and bacteriopheophytin a into large oligomers with maximum optical absorption at 860 nm was studied in a 3:1 (vol/vol) formamide/water solution, using optical absorption spectroscopy and electron microscopy. The aggregation is cooperative and proceeds according to two equilibrium constants. Initially, two pigment molecules form a “seed” that absorbs at ≈860 nm. The equilibrium constant, K(a), governing this reaction equals 1.3 × 10(3) M(-1) in the case of bacteriochlorophyll a (due to experimental limitations, K(a) for bacteriopheophytin a could not be determined). The addition of monomers to aggregates consisting of two or more units is governed by an equilibrium constant, K(b), equal to 2.2 × 10(6) M(-1) for bacteriochlorophyll a and ≈ 10(9) M(-1) for bacteriopheophytin a. The enthalpy and entropy changes that drive the bacteriochlorophyll oligomer formation are -9.25 and ≈0.0 kcal/mol, respectively. Above a threshold concentration, the amount of oligomers remains constant but their length continues to increase. Each oligomer appears to consist of dimers that are associated by hydrophobic interactions among their alcohol residues, forming long strands. Single strands presumably coil into helices that are seen as cylinders. The bacteriochlorophyll a oligomers form cylinders with a constant diameter of 150 Å and an average length of 2,000 Å (at 1.5 × 10(-5) M bacteriochlorophyll a). These cylinders contain 200-250 bacteriochlorophyll a dimers. The bacteriopheophytin oligomers coil into wider cylinders (≈400 Å in diameter) which contain ≈600-700 bacteriopheophytin a dimers. In both cases, the separation between the dimers is ≈20 Å. At such distances, the dipolar interactions among adjacent dimers are negligible and do not affect the optical absorption of each individual pair. Therefore, the optical absorption of these pairs can be a tool for investigating the absorption pattern of photosynthetic pigments in vivo