azathioprine

The story

1. Proto-Indo-European*né
2. Proto-Indo-European*n̥-
3. Proto-Hellenic*ə-
4. Ancient Greekᾰ̓- (ă-)
5. Proto-Indo-European*gʷeyh₃-
6. Proto-Indo-European*gʷih₃wós
7. Proto-Indo-European*gʷíh₃weti
8. Proto-Hellenic*gʷyṓw-ō
9. Proto-Hellenic*d͏̌ṓwō
10. Ancient Greekζώω (zṓō)
11. Ancient Greekζῶ (zô)
12. Ancient Greekζωή (zōḗ)
13. Frenchazote
14. Englishazote
15. Englishaza-
16. Proto-Indo-European*dʰewh₂-der.?
17. Ancient Greekθέειον (théeion)
18. Ancient Greekθεῖον (theîon)
19. Englishthio-
20. GermanPurin
21. Englishp(u)rine
22. English todayazathioprine

From aza- + thio- + p(u)rine.