Table 2. Porphyrin synthesis in tumour explants incubated with ALA and ALA derivatives.

	μg porphyrin/g tissue
Control	0.33±0.02
ALA	5.17±0.26
He-ALA	5.83±0.38
Undecanoyl-ALA	1.18±0.07
THP-ALA	5.53±0.27

Tumour explants were incubated for 3 h with 0.6 mM ALA or ALA derivatives. Porphyrins were extracted and determined as described in Materials and Methods. Controls correspond to the tissue basal porphyrin levels.