Spiro-oxindole scaffolds have been studied due to their promising therapeutic potential. In the Amazon rainforest there are two important Uncaria species known as “cat’s claw”, which biosynthesize spirocyclic oxindole alkaloids; Uncaria tomentosa (Willd. ex Schult.) DC. and Uncaria guianensis (Aublet) Gmell. We carried out a precursor-directed biosynthesis approach with U. guianensis and successfully obtained oxindole alkaloid analogues with molecular mass corresponding to the addition of a methyl or fluorine group on the oxindole ring using tryptamine analogue precursors. Two of these novel oxindole alkaloid analogues (3b-7-methyl-isomitraphylline and 3c-6-fluoro-isomitraphylline) were isolated and characterized by NMR spectroscopy and ESI-QTOF-MS/MS. Having established a substrate feeding protocol for these plantlets, the biosynthetic route for mitraphylline (1), rhynchophylline (2), isomitraphylline (3) and isorhynchophylline (4) was also investigated using ¹³C-precursors (1-¹³C-D-glucose, 2-¹³C-tryptophan, 1-¹³C-DL-glyceraldehyde, and methyl-¹³C-D-methionine).