Actinomycetes are valuable soil and marine microorganisms known to produce diverse secondary metabolites including antibiotics, antitumor agents, immunosuppressants, antiparasitic agents, and enzyme-inactivating compounds with potent antibacterial, antifungal, neuritogenic, anticancer, antialgal, antimalarial, and anti-inflammatory biological activities. Among these secondary metabolites are the lorneic acids (A-K), trialkyl-substituted aromatic acids possessing a unique carboxylic side-arm and conjugated double bond. In 2009, lorneic acids A and B were extracted from an actinomycetes strain (NPS554) isolated from a marine sediment sample collected in Miyazaki Harbor, Japan. Initial biological investigations revealed that lorneic acid A exhibits significant inhibitory activity against phosphodiesterases (PDE) with selectivity for PDE type-5 (PDE5) inhibition. Lorneic acid B showed weaker PDE5 inhibition, suggesting the functional importance of the conjugated double bond found in the side-chain of lorneic acid A. PDE5 inhibitors, widely recognized for their treatment of erectile dysfunction and pulmonary hypertension, have shown to be anticancer, cardioprotective, and useful against neurodegenerative diseases, suggesting these lorneic acids as potential therapeutic candidates.