Effects of light fractionation and different fluence rates on photodynamic therapy with 5-aminolaevulinic acid in vivo

Abstract

To improve efficacy of photodynamic therapy (PDT) with intravenously administered 5-aminolaevulinic acid (ALA) fractionating the light dose or reducing the light intensity may be a possibility. Therefore, Syrian Golden hamsters were fitted with dorsal skinfold chambers containing an amelanotic melanoma (n=26). PDT was performed (100 mW cm(-2), 100 J cm(-2), continuously or fractionated, and 25 mW cm(-2), 100 J cm(-2); continuously or fractionated) using an incoherent light source following i.v. application of ALA. Following fractionated irradiation, the light was paused after 20 J cm(-2) for 15 min. Prior to and up to 24 h after PDT tissue, pO(2) was measured using luminescence lifetime imaging. The efficacy was evaluated by measuring the tumour volume of amelanotic melanoma cells grown subcutaneously in the back of Syrian Golden hamsters (n=36). Only high-dose PDT resulted in a significant decrease of pO(2). Irrespective of the mode of irradiation only high-dose PDT induced complete remission of all tumours (13 out of 13). It could be shown that low-dose PDT failed to induce a significant decrease of pO(2). No significant effect of fractionated irradiation was shown regarding the therapeutic efficacy 28 days after PDT. Thus performing a fractionated PDT with ALA or reducing the light intensity seems not to be successful in clinical PDT according to the present data.

Figure 1

Oxygen maps of the dorsal skinfold chamber with A-Mel-3 tumour prior to and after PDT (100 mW cm⁻², 100 J cm⁻², continuous irradiation) over the time (prior to, 30 min, 2 h and 24 h after PDT). The maps are pseudocolour images, the colour bar gives information regarding the colour–pO₂ relation. Prior to PDT, the tumour region can be clearly differentiated from normal tissue because of its blue colour due to the lower oxygen tension. The pO₂ is reduced after irradiation in tumour and surrounding tissue.

Figure 2

pO₂ was measured prior to and 15 min, 1 h (30 min), 2 h and 24 h (if fractionated additionally in the light pause) after high-dose PDT (100 mW cm⁻², 100 J cm⁻²) in tumour tissue (▪) and in surrounding tissue (□) after continuous (A, n=7) or fractionated (B, n=6) irradiation. A decrease of pO₂ in tumour and surrounding tissue is shown following high-dose PDT (median±s.e.m.).

Figure 3

pO₂ was measured prior to and 15 min, 1 h, 2 h and 24 h (if fractionated additionally in the light pause) after low-dose PDT (25 mW cm⁻², 100 J cm⁻²) in tumour tissue (▪) and in surrounding tissue (□) after continuous (A, n=7) or fractionated (B, n=6) irradiation (median±s.e.m.).

Figure 4

A-Mel-3 amelanotic melanomas were implanted subcutaneously in the dorsal skinfold of Syrian Golden hamsters (n=38). At 150 min after i.v. ALA application (500 mg kg⁻¹ b.w.) irradiation was performed. Six groups were formed according to the protocols: (♦) control (I, n=6); (◂) high-dose PDT (100 mW cm⁻², 100 J cm⁻²), continuous irradiation (II, n=7); (▪) high-dose PDT (100 mW cm⁻², 100 J cm⁻²), fractionated irradiation (III, n=6); (•) low-dose PDT (25 mW cm⁻², 100 J cm⁻²), continuous irradiation (IV, n=7); (▴) low-dose PDT (25 mW cm⁻², 100 J cm⁻²), fractionated irradiation (V, n=6); (▾) low-dose PDT (25 mW cm⁻², 100 J cm⁻²), continuous irradiation without ALA (VI, n=6)]. Tumour volumes were recorded throughout the complete observation period (28 days) (median±s.e.m.).