Why on earth would you consider it too large to cross the BBB? Both fit within pharmacologically active mass profiles of small molecules, and even fit
decently towards suggested drug lead characteristics, which are more stringent. I don't mean this insultingly, but it sounds to me as though as you
just aren't yet educated about such things. I'm also assuming you mean mass when referencing molecular size, so please correct me if you mean surface
area. That gets slightly more nuanced.
Heroin, an example I will continue with later, is a larger opioid and obviously passes. Passing the BBB, or not, or being pumped out depends on a few
factors. One is whether your molecule is actively transported (often p-glycoprotein substrates, as a prototypical example, though there are various
transporters), and another feature that is paramount is the lipophilicity of the drug, since this allows a drug to pass cell membranes rather than
remain in the blood stream.
This is why acetylating morphine twice to heroin increases BBB permeability despite adding bulk to the molecule. De-acetylation on the brain side can
then trap the morphine, by the way. A somewhat analogous aspect with your title compound is the ether formation of the 3-OH, which negates hydrogen
bonding and acidity from the alcohol in dextrorphan. DXM has a predicted logP within 2.9-3.3 vs. DXO's 3.5-3.75. LogS follows expected trends (-3.2
compared to -4.5, respectively). This is all typically far more of a pharmacology topic than even biochemistry, for anyone curious.
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