Disclosed is the use of a compound of Formula III e.g. 2,3,4,7,8,9-hexahydro-1H-cyclopenta[h]quinolin-2-one or a pharmaceutically acceptable derivative thereof as hereinbefore defined, for treating a disease involving a-synuclein toxicity and/or fibril formation, inhibiting or preventing a-synuclein fibril growth, and/or causing disassembly, disruption, and/or disaggregation of n-synuclein fibrils and a-synuclein-associated protein deposits e.g. Parkinsons disease.