Dual inhibitor synergy model

I am looking for an appropriate design and modeling strategy for inhibition of levodopa-induced dyskinesia (LID) in a rat model of Parkinsons disease. Dyskinesia (AIM) is scored categorically 0,1,2,3 or 4. The two active drugs by them selves only have a minor maximum response in the model (reversal of AIMS), but displays synergy when combined. My intended PKPD study design is maintaining a relative constant dyskinesia level with levodopa, and combine three doses each (low, medium, high) of the inhibitory drugs A and B, in a total of 9 treatments in addition to a levodopa baseline study. With respect to the modeling strategy, my intention is to establish a baseline model for levodopa-induced dyskinesia. Do you have suggestions/literature references for models that could be applied to analyse the synergistic inhibitory responses of drug A and B? Best regards, Mads