Genotoxicity and cytotoxicity of oxindole alkaloids from Uncaria tomentosa (cat's claw): Chemotype relevance

Uncaria tomentosa (Willdenow ex Roemer & Schultes) DC. (Rubiaceae) or cat's claw is a climber vine from the South American rainforest used in folk medicine for cancer treatment. Its antitumor activity has been mostly ascribed to pentacyclic oxindole alkaloids (POA) from stem bark and leaves while the activity of tetracyclic oxindole alkaloids (TOA) remains unknown. In recent times, the occurrence of three chemotypes based on its oxindole alkaloid profile was noticed in U. tomentosa, namely, chemotype I (POA cis D/E ring junction); chemotype II (POA trans D/E ring junction) or chemotype III (TOA). Consequently, the relationship between the chemotype and cytotoxic and genotoxic activities deserves attention. To evaluate the influence of cat's claw chemotypes on genotoxicity and cytotoxicity against non malignant and malignant human cell line models. Four authentic stem bark cat's claw samples (SI-SIV) and two leaf samples (LII and LIII) were analyzed by HPLC-PDA, properly extracted and fractioned by ion-exchange to obtain oxindole alkaloid purified fractions (OAPFs). The freeze-dried fractions were assayed for genotoxicity and cytotoxicity against human leukocytes (non malignant cell line) by the micronuclei frequency method and the alkaline comet DNA assay, and the trypan blue method, respectively. Moreover, the cytotoxicity of each OAPF was evaluated against a human bladder cancer cell line (T24) and human glioblastoma cell line (U-251-MG) by MTT method (malignant cell lines). Additionally, the isomerization of oxindole alkaloids throughout the course of cell incubation was monitored by HPLC-PDA. Based on HPLC-PDA analyses, sample SI was characterized as chemotype I, while samples SII and LII were characterized as chemotype II, and samples SIII, SIV and LIII as chemotype III. The chemotypes showed comparable cytotoxic activity toward malignant cell lines (T24 and U-251-MG) unlike human leukocytes (non malignant cell line), where this activity was clearly distinct. Chemotype II (POA trans D/E ring junction) showed a higher selectivity index (SI) against malignant cells (SI=1.11–3.04) than chemotype I (SI=0.10−0.19) and III (SI=0.21–0.57). No important genotoxic potential was found by micronuclei frequency and alkaline comet DNA assays. Despite the isomerization of oxindole alkaloids during the cell incubation, the chemotype of the cat's claw samples remained unchanged. Cat's claw chemotypes showed different selectivity against human malignant cells, so that