In recent years, proteolysis-targeting chimeras (PROTACs) have shown tremendous promise in targeting "undruggable" proteins. By simultaneously engaging a target protein and an E3 ubiquitin ligase, PROTACs trigger selective protein degradation and enable precise removal of disease-driving proteins.

However, PROTACs are typically large and highly polar, with molecular weights often exceeding 800 Da---far beyond Lipinski's Rule of Five (Ro5) for orally bioavailable small-molecule drugs. As a result, they are widely believed to suffer from poor membrane permeability and inefficient cellular uptake---mechanisms that remain poorly understood. Overcoming these limitations has become a key challenge in medicinal chemistry.

A collaborative study from The University of Texas Health Science Center, Duke University, University of Arkansas and others reports that CD36 (cluster of differentiation 36) is a membrane receptor that mediates cellular uptake of PROTACs and other "beyond-Rule-of-Five" (bRo5/eRo5) molecules.

Using biotin-probe profiling, gene knockout/knock-in models, UPLC-MS metabolomics and computational simulations, the researchers demonstrated that:

• CD36 binds multiple PROTAC molecules (including SIM1-Me, MZ1 and ARV-110) as well as large-molecule drugs such as rapamycin, navitoclax, birinapant, tubacin and doxorubicin, thereby promoting their cellular uptake and pharmacological activity.

• Loss of CD36 drastically reduces drug uptake and target protein degradation, whereas restoring CD36 expression rescues PROTAC efficacy.

• CD36-mediated drug internalization depends on classical endocytosis via the Rab5/EEA1 early endosome pathway.

• Structural optimization that enhances PROTAC affinity for CD36 can significantly improve permeability and activity.

Through membrane proteomics screening using biotin-labeled PROTAC probes, the team identified CD36 as a major binding partner of both CRBN- and VHL-based PROTACs, as well as several non-traditional small-molecule therapeutics such as rapamycin and doxorubicin.

This work reveals that CD36 is not limited to fatty-acid transport---its broad ligand recognition ability positions it as a potential "drug-transport receptor."

Using shRNA to silence CD36 in LNCaP prostate cancer cells, the researchers found that treatment with SIM1-Me (10 nM) or ARV-110 (50 nM) led to:

• 5.6-fold and 19.6-fold reductions, respectively, in cellular drug uptake

• dramatically impaired target protein degradation

• markedly reduced cytotoxicity (up to 423-fold)

Re-expression of CD36 restored activity, and loss of EEA1 also diminished SIM1-Me and ARV-110 cytotoxicity---confirming a dependence on the Rab5/EEA1 endocytic pathway.

Similar effects were also observed with representative bRo5 molecules in TNBC and prostate cancer cells.

To increase affinity toward CD36, researchers modified the linker region of the VHL-based PROTAC MZ1 by introducing cleavable polar groups (e.g., carboxyl, amino, Boc-protected groups). These "prodrug-like" derivatives---such as MZ1-C14-Na and MZ1-C12-NB---showed

• 22.3-fold and 7.7-fold increases in intracellular accumulation vs. MZ1

• superior tumor-killing activity

• rapid intracellular conversion back to active MZ1 (>60% within 1 hr)

This demonstrates that tuning CD36 affinity offers a feasible strategy to improve permeability of PROTACs and other large molecules.

CD36 is now recognized as a major receptor facilitating cellular uptake of PROTACs, divalent inhibitors and high-molecular-weight drugs (543--2145 Da). Screening for receptor-mediated uptake (e.g., via CD36) should therefore be incorporated into medicinal chemistry workflows.

The discovery of CD36 as a "universal carrier" suggests that medicinal chemists can rationally enhance CD36 binding to improve intracellular exposure. This may become a broadly applicable strategy for PROTAC and bRo5 drug design---although further validation is required.

Mutations or loss of CD36 may contribute to reduced PROTAC efficacy or resistance. Additionally, clinically advancing PROTACs may benefit from optimization toward receptor-mediated uptake.

This groundbreaking Cell publication is the first to establish CD36 as a key endocytic receptor for bRo5-class drugs and to propose a generalized "CD36-mediated chemical endocytosis" strategy. The findings greatly expand the theoretical and practical foundation for large-molecule drug discovery.

As structurally complex therapeutics continue to emerge, this work offers a powerful "key" to unlock the cell membrane---paving the way for next-generation PROTACs, targeted therapies and precision oncology.

However, the current study was conducted primarily in prostate and breast cancer cell lines; whether CD36 is universally required for PROTAC uptake across other cell types remains to be further explored.