Research ArticleGPCR SIGNALING

Biased antagonism of CXCR4 avoids antagonist tolerance

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Science Signaling  16 Oct 2018:
Vol. 11, Issue 552, eaat2214
DOI: 10.1126/scisignal.aat2214

The right kind of bias

AMD3100, an antagonist of the chemokine receptor CXCR4, prevents the accumulation of leukemic cells in the bone marrow, which promotes the efficacy of chemotherapeutic agents. However, tolerance to AMD3100 can develop, which leads to receptor accumulation on the cell surface and retention of cells in the bone marrow. Hitchinson et al. showed that a peptide derived from CXCR4 avoided the development of tolerance by acting as a biased antagonist of G protein signaling but not β-arrestin–mediated internalization of CXCR4. Similar properties were shared by a nonpeptide, small-molecule inhibitor that also did not stimulate tolerance. These results suggest that the use of biased GPCR antagonists could be of therapeutic benefit in patients who have developed tolerance to nonbiased antagonists.


Repeated dosing of drugs targeting G protein–coupled receptors can stimulate antagonist tolerance, which reduces their efficacy; thus, strategies to avoid tolerance are needed. The efficacy of AMD3100, a competitive antagonist of the chemokine receptor CXCR4 that mobilizes leukemic blasts from the bone marrow into the blood to sensitize them to chemotherapy, is reduced after prolonged treatment. Tolerance to AMD3100 increases the abundance of CXCR4 on the surface of leukemic blasts, which promotes their rehoming to the bone marrow. AMD3100 inhibits both G protein signaling by CXCR4 and β-arrestin1/2–dependent receptor endocytosis. We demonstrated that biased antagonists of G protein–dependent chemotaxis but not β-arrestin1/2 recruitment and subsequent receptor endocytosis avoided tolerance. The peptide antagonist X4-2-6, which is derived from transmembrane helix 2 and extracellular loop 1 of CXCR4, limited chemotaxis and signaling but did not promote CXCR4 accumulation on the cell surface or cause tolerance. The activity of X4-2-6 was due to its distinct mechanism of inhibition of CXCR4. The peptide formed a ternary complex with the receptor and its ligand, the chemokine CXCL12. Within this complex, X4-2-6 released the portion of CXCL12 critical for receptor-mediated activation of G proteins but enabled the rest of the chemokine to recruit β-arrestins to the receptor. In contrast, AMD3100 displaced all components of the chemokine responsible for CXCR4 activation. We further identified a small molecule with similar biased antagonist properties to those of X4-2-6, which may provide a viable alternative to patients when antagonist tolerance prevents drugs from reaching efficacy.

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