proc nlmixed;
parms b1=298 b2=70 b3=-.2 b4=0.89 b5=-.2 s2=310 ;
pred=b1 + D*(b2*(1-exp(b3*DoseDLM))) + H*(b4*(1-exp(b5*DoseHMTBA)));
model ADG~normal(pred, s2);
estimate 'PLATEAU DIFF' b4-b2;
run;

http://ps.fass.org/cgi/reprint/85/5/947.