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CXCR4 antagonists, such as the small molecule inhibitor AMD3100, mobilize hematopoietic stem/progenitor cells (HSPC) with rapid kinetics, but rather low efficiency, when used alone.1 Mobilization with AMD3100 can be improved several-fold if it is administered as continuous infusion,2 but, in addition to being expensive, this approach may be no more practical than serial G-CSF injection. We have shown that, like AMD3100, blockade of the α4-integrin alone also results in HSPC mobilization in mice, macaques and humans. However, it is of marginally sufficient potency for single-modality mobilization.3;4 As intolerance or resistance to G-CSF may require alternative mobilization regimes, we tested the efficiency of mobilization by a combination of α4-integrin blockade and AMD3100 in macaques and mice. Treatment of a macaque with a 4 mg/kg bolus of AMD3100 increased circulating CD34+ cells from <1/μl to 14/μl within 6 hours (Fig. 1A, left panel). I.v. administration (5 mg/kg) of the anti-functional α4-integrin antibody previously used in humans (natalizumab),3 on its own mobilized 5 CD34+ cells/μl after 48 hours. However, with a subsequent s.c. injection of 4 mg/kg AMD3100 48 hours after the natalizumab treatment, 27 CD34+ cells/μl were in circulation 6 hours later (Fig. 1A, middle panel), i.e. mobilization with the two modalities together was more than additive. Circulating CFU-C were increased accordingly; the frequency of CFU-C among CD34+ cells was approximately 1:10 at all time points. We furthermore demonstrate that, similarly to what we recently showed in mice,2 continuous infusion of AMD3100 of a dose of 10 mg/kg*day resulted in progressive accumulation of markedly higher HSPC numbers than can be mobilized by single-bolus injection (23 CD34+ cells/μl), although peak values may not have been reached after three days (Fig. 1A right panel). To corroborate these observations and to alleviate concerns about off-target effects of anti-functional antibodies, we tested mobilization in a genetic model of α4 deficiency. AMD3100 was injected i.p. at a maximally effective dose of 4 mg/kg,2;5 or by continuous s.c. infusion of 40 mg/kg*day for 5 days.2 α4-deficient mice had the expected high numbers of circulating CFU-C at baseline.6 Injection (Fig. 1B) or infusion (Fig. 1C) of AMD3100 resulted in mobilization of significantly greater numbers of CFU-C in the α4-deficient mice (mobilization of 1,000 CFU-C/ml over baseline with bolus, of 35,000 CFU-C/ml over baseline in infusion-treated animals) than in WT animals (mobilization of 700 and 10,000 CFU-C/ml over baseline for bolus and infusion, respectively), i.e. as in the macaque, mobilization was at least additive. These data have implications from both a practical and a mechanistic standpoint. First, we demonstrate the efficient mobilization by combination of α4- and CXCR4-blockade. In this respect, small-molecule inhibitors of α4-integrin, which were recently tested in mice, thus hold some promise for future clinical studies.7 Second, our data suggest that blockade of α4-integrin and of CXCR4, molecules which were both shown to be important for homeostatic retention of HSPC, mobilize through independent molecular mechanisms. Although additive to synergistic mobilization was also described for treatment with G-CSF+AMD3100,2;5 the mechanism of the co-operative function in this case is not clear and likely a different one. Since the AMD3100 was administered at the end of a 5-day course of G-CSF, the benefit of that combination may rely on the expansion of the target population for AMD3100. In contrast, α4-integrin blockade is not associated with significant proliferation.
Funding: Deutsche Krebshilfe (HB), NIH (KHC: DK077864-02, HPK: HL53750, AI061839, HL53750, TP: HL58734)
Author contribution summary:HB Conception and design, Financial support, Collection and/or assembly of data, Data analysis and interpretation, Manuscript writing, Final approval of manuscript
KLW Conception and design, Collection and/or assembly of data, Final approval of manuscript
KHC Collection and/or assembly of data, Final approval of manuscript
HPK Financial support, Provision of study material or patients, Final approval of manuscript
TP Conception and design, Financial support, Data analysis and interpretation, Manuscript writing, Final approval of manuscript