Madagascar periwinkle. Flowers of the Madagascar periwinkle (Catharanthus roseus or Vinca rosea). This plant is the source of the alkaloids vinblastine (VLB) and vincristine (VCR). These are used as chemotherapy agents in the treatment of certain cancers. VCR and VLB are always used in conjunction with other drugs. Both alkaloids are believed to interfere in the process of mitosis (cell division) by damaging the mitotic spindle, the apparatus of cell division. Both compounds have a short-lived toxicity; VCR towards the nervous system and VLB to the bone marrow.