and, surprisingly, found AP24163 demonstrated reasonable inhibitory action on top of potent inhibition of native Bcr-Abl. Following up on that breakthrough Ariad began further research to increase the efficacy of compound AP24163 against the T315I mutation. Docking of the molecule into the ATP binding site of T315I mutated Bcr-Abl kinase revealed that the expected steric clash with isoleucine was not present due to a lesser sterically demanding vinyl linkage between the purine core and the diarylamide side chain compared to other TKIs. The first step was to try to find an even less sterically demanding structure. First an acetylene linkage was tested, that resulted in higher potency but unfavorable pharmacokinetics. Later, a more stable 2-butyne linkage was selected. To achieve this linkage an imidazol[1,2-a]pyridine core was used as a starting material for a Sonogashira reaction; but the pharmacokinetics were still poor. When developing AP24163, adding a cyclopropane side chain on C8 in the purine core resulted in favorable pharmacokinetics. Several different side chains were then tested, but the best results were obtained with no side chain at all, resulting a substance with satisfactory pharmacokinetics, but now with reduced potency against T315I also. The first step in increasing the potency again was to look at other TKI's. Imatinib has a terminal methyl piperazine group which has been shown to form a hydrogen bond with the carbonyl oxygen atom of residue Ile-360 in the activation loop of the Abl kinase. The piperazine ring is also a common solubilizing group that could further improve the pharmacokinetic properties of the molecule. Those speculations were confirmed with a two-fold increase in inhibitory action against Bcr-Abl T315I mutated kinase and the silver lining was the plasma protein binding of the substance (named '19a') appeared to have decreased, allowing for smaller doses with the same potency. Whilst '19a' exhibited good oral pharmacokinetics in both mice and rats, it also retained high partition coefficient at 6.69. So, in attempts to reduce the molecule's lipophilicity further, substitution of a single carbon atom on the imidazo[1,2-a]pyridine core was made; which resulted in what is now known as the compound ponatinib. Binding X-ray crystallographic analysis of ponatinib and T315I Bcr-Abl mutated kinase display that the imidazo[1,2b]pyridazine core rests in the adenine pocket of the enzyme. The methylphenyl group occupies a hydrophobic pocket behind I315, the ethynyl linkage forms favorable van der Waals interactions with the amino acid and the trifluoromethyl group