Stability studies of HIV-1 Pr55(gag) virus-like particles made in insect cells after storage in various formulation media

BACKGROUND: HIV-1 Pr55(gag) virus-like particles (VLPs) expressed by baculovirus in insect cells are considered to be a very promising HIV-1 vaccine candidate, as they have been shown to elicit broad cellular immune responses when tested in animals, particularly when used as a boost to DNA or BCG vaccines. However, it is important for the VLPs to retain their structure for them to be fully functional and effective. The medium in which the VLPs are formulated and the temperature at which they are stored are two important factors affecting their stability. FINDINGS: We describe the screening of 3 different readily available formulation media (sorbitol, sucrose and trehalose) for their ability to stabilise HIV-1 Pr55(gag) VLPs during prolonged storage. Transmission electron microscopy (TEM) was done on VLPs stored at two different concentrations of the media at three different temperatures (4°C, –20°C and −70°C) over different time periods, and the appearance of the VLPs was compared. VLPs stored in 15% trehalose at −70°C retained their original appearance the most effectively over a period of 12 months. VLPs stored in 5% trehalose, sorbitol or sucrose were not all intact even after 1 month storage at the temperatures tested. In addition, we showed that VLPs stored under these conditions were able to be frozen and re-thawed twice before showing changes in their appearance. CONCLUSIONS: Although the inclusion of other analytical tools are essential to validate these preliminary findings, storage in 15% trehalose at −70°C for 12 months is most effective in retaining VLP stability.