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BACKGROUND: Vasoconstriction has been an obstacle to clinical development of Hb-based O2 carriers. It is proposed that this limitation can be overcome by increasing molecular size and oxygen affinity.

STUDY DESIGN AND METHODS: Surface-modified Hb (MP4) was designed, whose properties are consistent with the theory that cell-free Hb engages autoregulatory vasoconstrictive responses to Hb diffusion in the plasma space (“facilitated diffusion”). Human Hb was modified by reaction first with 2-iminothiolane to add sulfhydryl groups and then with monofunctional maleimide- activated 5-kDa PEG.

RESULTS: MP4 was found to have a molecular weight of 90 kDa, a molecular radius increased relative to native Hb (9.3 ± 1.4 vs. 3.2 nm), high oxygen affinity (p50∼ 5-6 mmHg), and a Bohr effect approximately half that of native human Hb (–0.24Δlogp50/ΔpH). At 4.2 g per dL in Ringer's lactate, its viscosity was 2.5 cP, and its oncotic pressure was 50 mmHg. The t50 of 14C-MP4 in rats was approximately 24 hours. No significant elevation in mean arterial pressure was observed.

CONCLUSION: MP4 appears to be free of a pressor effect, a major limitation to the development of a safe and effective RBC substitutes in the past.