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Advanced Neurology Brain bioavailability of targeted protein degraders

not readily traverse through the BBB, and its complexation facilitates the oral absorption of xenobiotics. OATP1A2 has
with a drug molecule makes this process even more difficult. a broad range of substrates, including bile salts and acids,
According to the “free drug theory,” drugs that extensively steroid conjugates, thyroid hormones, and xenobiotics such
bind to plasma proteins typically have lower CNS as methotrexate, fexofenadine, erythromycin, imatinib,
bioavailability. 125,126 In other words, high protein binding lopinavir, and some β-blockers. The high molecular weight
is undesirable for molecules targeted at brain indications, dopamine D2 receptor agonist bromocriptine (654 Da)
as a lower free fraction (f ) would be available for BBB is a substrate of OATP1A2. OATP2B1 is localized in the
u
permeation. However, there are notable exceptions, such as sinusoidal membranes of hepatocytes and enterocytes
long-acting, non-steroidal anti-inflammatory agents, such and the apical membrane of endothelial cells of the BBB.
as isoxicam and meloxicam, tricyclic antidepressants such OATP2B1 substrates include statin, prostaglandin E2,
as imipramine and desipramine, calcium channel blockers, thyroid hormones, and sulfonylureas. Although the
such as isradipine and darodipine, and benzodiazepine crystal structures of OATP1A2 and OATP2B1 are not
antagonists like flumazenil and iomazenil. 127-130 available, the crystal structure of OATP1B1 is available
from the Protein Data Bank. OATPs share a conserved
TPDs, in general, exhibit high plasma protein binding transmembrane helix fold that harbors a druggable
and non-specific binding, resulting in very low f . Therefore, binding pocket. Given that a significant number of TPDs
137
u
to meet the regulatory requirement of reportable f values have been reported to be orally bioavailable, the likelihood
u
≥0.01 for drug-drug interaction prediction, the dilution of TPD binding with OATP1B1 can be assessed though
method is commonly used to estimate the extent of plasma docking studies using SwissDock. OATP1A2 shares ~58%
protein binding for TPDs. In this approach, a diluted f , or identity with OATP1B1, and OATP2B1 shares ~68%
u
f ≥0.01, can be obtained by adjusting the dilution factor, identity with OATP1B1. The sequence identity is spread
ud
taking care not to saturate the potential binding sites. This across the entire sequence and is not concentrated in any
dilution method provides f levels comparable to those specific region. Therefore, docking studies with OATP1B1
u
estimated by the pre-saturation approach without plasma may provide a reasonable estimate of TPD’s likely affinity
dilution. The equilibrium is achieved rapidly, and the toward OATP1A2 and OATP2B1.
impact of non-specific binding is reduced. 131,132
In the case of TPDs, the percentage of unbound f in 12. Cereblon ligands and linker chemistry
u
plasma does not appear to correlate reasonably with the to augment BBB permeability
concentrations quantified in the brain. Various mechanisms While the protein-of-interest (POI) ligand depends on the
may contribute to this phenomenon. One possibility is target, there is a certain degree of flexibility in selecting E3
the interaction of the TPD-bound protein with the brain ligase ligands, such as CRBN, VHL, integrin-associated
capillary walls, leading to conformational changes and protein, and mouse double minute 2 homolog, as well
the release of the TPD from the protein. 133-135 Another as the linker chemistry. Furthermore, within each class
probability is protein-mediated transport, similar to what of E3 ligase ligands, there are various options to choose
is observed with neutral and basic drugs, such as diazepam, from. For example, in the case of CRBN, one can choose
disopyramide, and chlorpromazine. Their binding to alpha- between phenyl glutarimide, chiral lenalidomide, phenyl
1-acid glycoprotein enhances BBB permeability. Yet dihydrouracil, or benzoimidazolone, depending on the
126
another plausible mechanism is the enhanced extraction stability, potency, and druggability of the TPD. Phenyl
of the drug-protein complex into CNS regions with more glutarimide, for example, offers good aqueous solubility,
permeable capillary endothelium. 127,136 Another potential cellular permeability, and in vitro plasma stability, but it
mechanism could involve transporter-induced shifts in lacks stability in hepatocytes. Regarding linker chemistry,
protein binding. In this scenario, the TPD may detach from rigidification of linkers (e.g., linear to piperidine, piperazine,
the protein through transporters present on the luminal and spiro) has been shown to improve the absorption,
membrane before it dissociates and achieves binding distribution, metabolism, and excretion (ADME) profile,
equilibrium. High protein binding affinity, thus, would such as aqueous solubility, cellular permeability, and
not restrict the TPD’s permeation into the brain, making plasma and hepatocyte stability. Polyethylene glycol (PEG)
it incorrect to use in vitro f values obtained at equilibrium linker’s exhibit improved aqueous solubility, while PEG
u
to predict the drug available for brain penetration. The two amide linkers tend to impart poor cellular permeability.
predominant drug transporters in the brain are OATP1A2 O-linked acetamide PEG displays good aqueous solubility
and OATP2B1. OATP1A2 (SLCO1A2) is expressed in but poor plasma and hepatocyte stability, as well as low
endothelial cells of the BBB and the apical (brush border) cellular permeability. Conversely, the conversion of
membrane of enterocytes in the duodenum, where it amide into alkyl linker results in poor aqueous solubility,

Volume 4 Issue 2 (2025) 64 doi: 10.36922/an.5140