The rationale was: carbon dioxide goes through cells' lipids and releases a proton inside once hydrated. Then, let's find another compound that's also not barred by lipids and releases even more protons. This led to 2,6-DHBA.

It overlooks that the very property that makes the selected compound a greater acidifier is also what makes it extraordinarily prone to deprotonate prematurely. The ability to pass through lipids is likely to be compromised with the early proton release. If it could be injected into the tumor, it would still be uncertain if it could be taken up by target cells.

Healthy Cancerous Extracellular pH 7.4 ↓6.5 Intracellular pH 7.1 ↑7.5

For direct internal acidification, it would be more fitting to seek weak rather than strong acids, those that can take advantage of the bump in transitioning from the extracellular to intracellular compartment (6.5↗7.5) to deprotonate the molecule where intended.