Seasonality effect on the composition of oxindole alkaloids from distinct organs of Uncaria tomentosa from the Caribbean region of Costa Rica

Uncaria tomentosa (Willd.) D.C. (Rubiaceae), commonly known as “Uña de Gato” or “Cat's Claw” is a tropical vine from the rainforest used in traditional medicine and spread through Central and South America, including Costa Rica. There is an increasing demand for medicinal extracts with biological activity attributed mainly to oxindole alkaloids (OA), where the ratio between tetracyclic (TOA) and pentacyclic oxindole alkaloids (POA) determines its feasibility for medicinal applications. The ratio is affected by distinct factors including the dynamics of environmental conditions during seasons. The purpose of the study was to assess the seasonality effect in oxindole alkaloids content in relation to plant organs from U. tomentosa grown in the Caribbean region of Costa Rica. Young leaves followed by mature leaves presented the highest amount of total OA during seasons; for these, isoryncophylline, pteropodine and isomitraphylline, were the predominant OA. The POA/TOA ratio of both leaf materials was nearly 1:1 (3.2 mg g⁻¹: 3.1 mg g⁻¹). Bark and root material showed a pentacyclic chemotype in all seasons with a ratio of 6:1 (6.7 mg g⁻¹: 1.3 mg g⁻¹) with pteropodine and isomitraphylline as the predominant POA. The POA content presented seasonality with a significant increase from rainy to dry season in young leaves, bark and roots. In contrast, TOA amount remained virtually unchanged in all plant parts. Humidity and temperature between the studied seasons were constant except for precipitation, reflecting that differences of water content had an effect in the POA amounts. Further studies of abiotic factors, like water stress, could explain the variation of POA content due to seasonality.