About DoLoop in NONMEM

Dear NonMEM users, I am constructing a transduction model to capture disease onset time-delay using NonMEM. Because I need to use too mamy compartments (means too much ODE) to handle this delay, I want to use DoLoop to save a little room. But I can not find a way to use it in $DES block. The block and error message are listed as below, $DES I=4 DOWHILE(I.LT.22) ;(I also tried Do I=5,22, same as DOWHILE) im1=I-1 DADT(I) = KT*(A(im1)-A(I)) I=I+1 ENDDO Error message: DOWHILE(I.LT.22) X THE CHARACTERS IN ERROR ARE: DO 438 THIS WORD IS NOT APPROPRIATE FOR THIS BLOCK OF ABBREVIATED CODE. I am very appreciated if anyone can tell me how to incorporate DoLoop into $DES block. Thanks a lot! Best regards, Liu, Dongyang, PhD, Postdoc Fellow Department of Pharmaceutical sciences, State University of New York at Buffalo. Tel(o):01-716-645-4840, Cell: 01-716-908-6644,