On the nature of the active component of haematoporphyrin 'derivative'.

The active component of the photochemotherapeutic drug 'haematoporphyrin derivative' was studied by laser flash photolysis. The triplet extinction coefficient of this species was determined using a method in which the calculated value is dependent on the assumed molecular weight. This value was then incorporated in the evaluation of the quantum yield of triplet by a comparative technique. Since the maximum possible value of this yield is unity, it is shown that the molecular weight of the active component is greater than or equal to 3000, so that, on average, the drug molecule must contain at least 5 porphyrin units.