The Blood-Brain Barrier (BBB) is like a military checkpoint between your bloodstream and your brain. It's made of tightly packed cells with no gaps — designed to keep toxins out. But it also keeps most drugs out. To cross the BBB, a drug needs to be: - Small enough to slip through - Fat-soluble (lipophilic) enough to pass through cell membranes - Not a substrate for the "escorts" that actively pump drugs back out Why it matters for MHS: Most CDK2 inhibitors in development don't penetrate the brain well. AL-605 was specifically designed to be brain-penetrant. This is a major advantage over CDK2 inhibitors developed only for cancer (where brain penetration isn't the goal). For MHS, brain penetration isn't optional — it's essential.