Sustained in vivo activity of recombinant bovine granulocyte colony stimulating factor (rbG-CSF) using HEPES buffer

The purpose of this study was to develop a long-acting injectable formulation of bG-CSF for veterinary use. However, in order to achieve sustained in vivo activity it was first necessary to stabilize the protein at the injection site. Preformulation studies, as well as literature, suggest that bG-CSF aggregates at neutral pH ranges (i.e., pH 6-8) and at temperatures of approximately 40 degrees C. Therefore, bG-CSF will not retain its activity for an extended period of time at the injection site. During this study we determined that HEPES buffer has a very significant impact on protein stability as well as on biological performance. Recombinant bovine granulocyte colony stimulating factor (rbG-CSF) was formulated in 1 M HEPES buffer for subcutaneous injection into cows. bG-CSF formulated in 1 M HEPES buffer resulted in sustained in vivo activity of bG-CSF compared to the "control" formulation (control formulation: 5% mannitol, 10 mM acetate buffer, 0.004% tween-80, pH 4). White blood cell (WBC) count was used as a marker to evaluate in vivo activity of the formulation. WBC numbers remained above a threshold value for only 24-30 h for the control formula. However, when bG-CSF was formulated in 1 M HEPES, the WBC remained above threshold for 3 days or 72 h. Formulating bG-CSF in 1 M HEPES at pH 7.5 also resulted in greater solution stability. This was surprising since bG-CSF is intrinsically not stable at neutral pH. The effect of 1 M HEPES on the T(M) (temperature at maximum heat flow on calorimetry scan) of bG-CSF was determined by microcalorimetry. In the absence of 1 M HEPES buffer the T(M) was 48 degrees C (onset approximately 40 degrees C), while bG-CSF formulated in 1 M HEPES buffer has a T(M) of 59 degrees C (onset approximately 50 degrees C). Similar organic buffers, such as MOPS, HEPPS, TES, and tricine, also resulted in improved solution stability as well as in sustained in vivo activity. The dramatic effect of these buffers on stability and biological performance of bG-CSF is not well understood. One hypothesis is that the electrostatic interaction between the zwitterionic form of these buffers and bG-CSF provides stabilization against denaturation.