Major alkaloid in periwinkle leaves is

The major alkaloid in periwinkle leaves is Vinblastine.
Vinblastine is the salt of an alkaloid extracted from a flowering herb commonly known as periwinkle. Vinblastine is cell cycle phase-specific, it binds to microtubular proteins in the mitotic spindle, thereby preventing cell division during metaphase.
Vinblastine is an anti-cancer ("antineoplastic" or "cytotoxic") chemotherapy drug. This medication is classified as a "plant alkaloid".