TAK-438 is a potassium-competitive acid blocker (P-CAB) that reversibly inhibits gastric H+, K+-ATPase, [1] [2] with ID50 values of 0.86 mg/kg to histamine-stimulated acid secretion in anesthetized rats [1].

Gastric H+, K+-ATPase has a key role in the final secretion step of gastric acid, transporting H+, via an electroneutral exchange of H+ for K+, into the secretory canaliculus in parietal cells [1].

In cultured gastric glands, TAK-438 treatment resulted in a longer and stronger acid formation inhibition. The inhibition effect of TAK-438 on acid secretion seemed to be associated with gastric parietal cell physiology. After cultured gastric glands were incubated with TAK-438 for 2 h and hence the incubation buffer was replaced with the CK buffer, the acid formation stimulated by forskolin slowly recovered, but the acid formation was inhibited immediately in a concentration-dependent manner [2].

In rats, 1-4, 5-8, and 9-12 h after the administration of TAK-438 at 3 mg/kg p.o., acid secretion stimulated by histamine was strongly inhibited. 24 to 27 h after administration of TAK-438, there was an inhibition rate of 40%, and this was a significant and sustained inhibition. In Heidenhain pouch dogs treated with doses of 0.1 to 1 mg/kg TAK-438 p.o., the acid secretion stimulated by histamine was inhibited dose-dependently, and this effect lasted for > 48 h. 1, 3, and 6 h after administration of 1 mg/kg TAK-438 completely inhibited the acid secretion stimulated by histamine [1].

References:
[1].  Yasunobu Hori, Jun Matsukawa, Toshiyuki Takeuchi, et al. A Study Comparing the Antisecretory Effect of TAK-438, a Novel Potassium-Competitive Acid Blocker, with Lansoprazole in Animals. Journal of Pharmacology and Experimental Therapeutics, 2011, 337:797-804.
[2].  Jun Matsukawa, Yasunobu Hori, Haruyuki Nishida, et al. A comparative study on the modes of action of TAK-438, a novel potassium-competitive acid blocker, and lansoprazole in primary cultured rabbit gastric glands. Biochemical Pharmacology, 2011, 81:1145-1151.