Erythromycin it is not stable on ethyl alcohol vehicle, without adjusting pH

I am trying to formulate a stable product for industrial pharmaceutical production, but I am encountering difficulties in the stability of erythromycin in the gel vehicle. I am trying with this formulation: Erythromycin 2% and 4%, BHT 0.05%-0.1%, Hydroxypropyl cellulose (HPC-H) 2.75%, Propylene glycol up to 5% (optional), and Ethyl alcohol 96%.
Unadjusted pH is around 9.

What are my options to stabilize the erythromycin so that it does not degrade to erythromycin B over time?