[ Eq(Derivative(x1, t), (1*k25*k15*k16*(1 - k17/k18) + (-1)*k25*k2*x1 + (-1)*k25*k3*x1*x3)/k25),
  Eq(Derivative(x2, t), (1*k25*(k4 + k5*x4**k6/(k7**k6 + x4**k6)) + (-1)*k25*k8*x2)/k25),
  Eq(Derivative(x3, t), (1*k25*k9*x2 + (-1)*k25*k10*x3)/k25),
  Eq(Derivative(x4, t), (1*k25*k11*x5*k12**k13/(k12**k13 + x1**k13) + (-1)*k25*k14*x4)/k25) ]