PK Model (NONMEM)

I have been working on a drug administerd orally to paediatric patient and follows a one compartmental model. Following is the control stream and the output obtained running the model with NONMEM VI. The estimates obtained do match the reported literature values. My next plan is to use the estimates obtained to model the PD data.

$PROB RUN# 100

$INPUT C ID TIME DV AMT DOSE MDV WT

$DATA 100.CSV IGNORE=C

$SUBROUTINES ADVAN2 TRANS2

$PK

TVCL=THETA(1)*(WT/70)**0.75

CL=TVCL*EXP(ETA(1))

TVV=THETA(2)*(WT/70)**1

V=TVV*EXP(ETA(2))

TVKA=THETA(3)

KA=TVKA*EXP(ETA(3))

S2=V
TAD=TIME SID=ID

$ERROR Y = F + F*ERR(1) + ERR(2)

IPRED=F

$EST METHOD=1 PRINT=20 NOABORT MAXEVAL=9999 MSFO=100.msf

$THETA

(0.1, 14) ;[CL/F]

(0.1, 51);[V]

(0.1, 8) ;[KA]

$OMEGA

0.09 ;[P] omega(1,1)

0.04 ;[P] omega(2,2)

0.36 ;[P] omega(3,3)

$SIGMA

0.09 ;[P] sigma(1,1)

0.1 ;[A] sigma(2,2)

$COVARIANCE
$TABLE ID SID TIME DV PRED IPRED CL V KA WT ONEHEADER NOPRINT FILE=100.tab
$$TABLE ID TIME IPRED WRES NOPRINT ONEHEADER FILE=SDTAB100

Output:

CL: 14.14 (37.7%), V:55.2 (20.7%), Ka: 8.12 (130%)

Residual Varaiability: Proportaional (15.4%), Additive (1.03 ug/ml)

Plots of few patients

Any comments/criticism/suggestions to improve the fit?