1. Conclusions An appreciable dose-concentration-response relationship between NN1731 and F 1+2 was expressed in a population PK/PD model. Since F 1+2 appearance traces the formation of thrombin, this relationship supports the possibility of using F 1+2 as a biomarker for haemostatic agents. Aims To explore the potential of prothrombin fragments 1&2 (F 1+2 ) as a biomarker for haemostatic agents with a model of the effects of the FVIIa analogue NN1731. Background Haemophilic patients suffer a defect in blood coagulation due to lack of either coagulation factor VIII or IX. In some patients, replacement therapy with the lacking coagulation factor eventually results in the formation of antibodies (inhibitors). Inhibitor patients may be treated with by-passing agents such as activated human coagulation factor FVII (FVIIa, NovoSeven  ). NN1731 is a FVIIa analogue that in vitro has shown increased activity in stimulating the cleavage of prothrombin to thrombin and F 1+2, a key step in the coagulatory pathway 1. F 1+2 was measured in the first clinical trial with NN1731, a dose escalation trial with 4 dosing arms. NN1731 and F 1+2 plasma concentrations were related to NN1731 doses to establish a population PK/PD model treating F 1+2 as a PD biomarker. A Population PK/PD Model Assessing the Pharmacodynamics of a Rapid- acting Recombinant FVIIa Analogue, NN1731, in Healthy Male Subjects. Andreas Groth 1, Judi Møss 2, Tine Møller 3, Steen H. Ingwersen 1 1 Biomodelling, 2 Medical and Science, NovoSeven Key Projects, 3 Biostatistics, Novo Nordisk A/S, Copenhagen, Denmark  avg@novonordisk.com Results Figure 3. Dose-independence check of PK model Post-hoc parameter estimates of CL and V 2 were checked for dose independence. Such a dependence appears to be absent for both parameters, indicating that the PK model is valid over the studied dose-range. Strategy: i.Develop PK-model from NN1731 plasma concentration data. ii.Develop PD-model from individual post-hoc PK model parameters and F 1+2 plasma concentration data. Data source: (3 pre- and 10 post-dose PK samples + 1 pre- and 5 post-dose PD samples)  6 healthy subjects  4 active (non-zero) dose levels. Dose range 5 µg/kg-30 µg/kg Modelling: PK and PD in man was modelled sequentially using NONMEM V with FOCE. Regarding inter-individual variability (i.i.v.) on model parameters, log-normal distributions were tested for significance against the hypotheses of zero i.i.v on that parameter (which is why the geometric, rather than the arithmetic, post-hoc estimate means are displayed on fig. 4). Methods Figure 4. Dose-independence check of PD model Figure 1. Structure of PK/PD model The resulting PK model was a standard two-compartment model with inter-individual variability (i.i.v.) on CL and V 2. The PD model was a linear indirect response model with the plasma concentration of NN1731 affecting the formation of F 1+2, incorporating i.i.v. on baseline F 1+2 levels ( B ) and the efficacy parameter ( E ). The F 1+2 formation rate at the baseline state C p =0 equals B  k out. F 1+2 B  k out  (1+ E  C p ) k out CL V1V1 V2V2 NN1731 dose Q CpCp NN1731 effect The individual post-hoc parameter estimates in the PD model were also checked for dependence on NN1731 dose. The result is less clear-cut than that of the PK parameters (fig. 3), but since the ranges of values for the lowest and the highest dose are quite similar for the efficacy parameter E, it is concluded that the NN1731 concentration-PD response relationship is well described. o Individual post-hoc estimates + Geometric mean of individual post-hoc estimates o Individual post-hoc estimates + Geometric mean of individual post-hoc estimates Figure 2. Fit of PK/PD model PK/PD model parameters V 1, V 2 : central & peripheral volumes of distribution, CL: clearance, Q: intercompartmental clearance, B: baseline F 1+2 level, k out : rate constant for F 1+2, E: NN1731 efficacy Values in parenthesis: Coefficients of Variation (CV’s) regarding i.i.v. for each parameter. The PK/PD model predictions of the F 1+2 time profiles for each trial subject as well as for the typical subject are shown along with the observations and their means for each dose level.,,.. Model predictions (individual) Model predictions (typical subject) o,o,.. Observations + Mean of observations at time point 5  g/kg 10  g/kg 20  g/kg 30  g/kg References 1 E. Persson et al, Proc Natl Acad Sci U S A, 96;13583,2001 NN1731 dose (µg/kg) NN1731 dose (µg/kg) NN1731 dose (µg/kg) NN1731 dose (µg/kg) E pM/h/(IU/ml) V 2 (ml/kg) CL ml/kg//h B (pM) Model parameter V1V1 V2V2 CLQBk out E Unit ml/kg ml/kg/h pM1/hpM/h/IU/ml) Estimate 59.678.0 (14%)120 (14%)38.6132 (29%)0.3460.164 (46%) S.E. of estimate 2.24.04.52.0110.0480.042 A sketch of the pro-coagulatory actions of thrombin, also known as coagulation factor IIa. Thrombin activates several coagulatory proteins and these actions cascade down eventually leading to the formation of cross-linked fibrin (CLIa) which forms the actual blood clot.