PK of i.d. and s.c. adalimumab administration
The adalimumab concentration time profile is displayed in Figure
3D . First, a non-compartmental analysis of pharmacokinetics (PK) was
performed. After exclusion of subjects where any leakage occurred during
injection, in the remaining subjects Cmax was
significantly higher after i.d. injection compared to s.c. injection
(90% CI 0.57-0.90, p=0.02). No difference was detected in
AUC0-inf (90% CI 0.55-1.09, p=0.22) or
AUC0-last (90% CI 0.60-1.07, p=0.20) (per protocol
subjects in Table 2, all enrolled subjects inSupplementary table 1 ). These data show that i.d.
administration of adalimumab yields a higher maximum concentration than
s.c. administered adalimumab.
To further examine PK and to be able to correct for inter-individual
variation in the kinetics of adalimumab and the formation of
anti-adalimumab antibodies, a population PK model was developed. After
exclusion of subjects in which any spill of adalimumab occurred during
administration, data from 10 s.c. and 9 i.d. injections was available
for model development using 275 adalimumab measurements that were above
the lower limit of detection (LOD). A total of 4% of the measurements
was below the LOD and therefore excluded from analysis. A significant
effect between the time-varying titre levels and the CL was identified
(p<0.001), indicating that the CL of adalimumab increases in
the presence of high titre levels. However, a bias in the absorption
kinetics for s.c. and i.d. was identified with linear absorption
kinetics. Subsequent exploration of different structural absorption
models resulted in a model event time (MTIME) function for the
absorption rate constant (ka) after i.d. administration
and two separate absorption compartments with equal kas
and one with an absorption lag time for s.c. administration to be best
fit for purpose (Figure 3E ). In this revised structural model,
significant (p<0.01) inter-individual variability on the
titre-CL relationship and the central volume of distribution was
identified. Additionally, a significant (p<0.01) improvement
in model fit was quantified after estimating a 29% higher
bioavailability (F) after i.d. administration of adalimumab compared to
s.c. administered adalimumab. A negative age-CL relationship and a
positive weight-CL relationship were identified. Both covariates gave
p<0.001 improvement in the model fit. The developed model
showed an overall accurate description of the absorption and elimination
phase of adalimumab (Supplementary figure 2A-B ). Model
parameters (Table 3 ) were estimated with high precision and
were comparable to literature values (23) . Simulations of the typical
adalimumab absorption rates over time showed a clear difference between
both administration routes, in which the i.d. dose had a fast initial
phase which decreased after MTIME, whereas the s.c. administration had a
slower initial phase and a small increase in the absorption rate,
approximately 2 hours after dosing (Figure 3F ).
Cytokine production was assessed by stimulating ex vivo whole
blood with LPS and aluminium hydroxide, driving NFκB and NLRP3
inflammasome activation. Results are shown in Figure 4 . Free
TNFα levels after both s.c. and i.d. administration sharply decreased
from pre-dose to post-dose (mean levels pre-dose i.d. 897 pg/mL, i.d.
48h post-dose 50 pg/mL, s.c. pre-dose 928 pg/mL, s.c. 48h post dose 74
pg/mL), as has been reported earlier (16), and returned to baseline at
the end of study (i.d. 70 days post-dose 1149 pg/mL, s.c. 70 days post
dose 850 pg/mL). No significant differences in inhibition of cytokine
release were detected when i.d. adalimumab administration was compared
to s.c. adalimumab administration (IFNγ p=0.61; IL-6 p=0.31; IL-8
p=0.81; IL-1β p=0.61; TNFα p=0.80). For LPS/aluminium hydroxide induced
IFNγ production after adalimumab administration, a gender effect has
been reported (14). A gender effect was not detected in this study (IFNγ
p=0.99, IL-6 p=0.80; IL-8 p=0.96; IL-1β p=0.75; TNFα p=0.08).