Bioequivalence and Bioavailability Forum

Hi Bob,

❝ I want to do non-compartmental analysis on plasma PK data […] Doses used in first and second cycle are same, however, the predose conc. for second cycle is 10. Concentration for first time point of second cycle being 10 …

OK. By design WinNonlin can calculate NCA for single dose data and for data in steady state. According to the superposition principle (H. Dost, 1953) nothing else makes sense.

❝ … (not 0 at 0 hr for second cycle) or >LLOQ although close to LLOQ (with Cmax of 200, last measured conc. of second cycle 10, achieving steady state), makes the baseline look like 10.

The definition when steady state is reached in practice is based on convention. Most people are happy with 5×t_½ (96.88% of steady state) whilst others are more strict (7×t_½ 98.44%, 10×t_½ 99.38%). In your case we have 95.00%. Fine with me. Since the dosing interval was less than 5×t_½ I leave it to you to judge whether that is sufficient. The concentration at the end of the second interval – which was identical to the first one – gives some confidence that no more relevant accumulation occurred.

❝ I want to know if NCA can be done directly on this data set from second cycle?

Yes.

❝ Or it may lead to error because of drug still being present in the system before second infusion?

That’s perfectly fine.

❝ I am primarily concerned about parameters including terminal t1/2, AUClast & inf, CL, MRT, Vss?

AUC_0–∞ in (pseudo) steady state does not make sense. Either AUC_0–∞ (single dose) or AUC_0–τ (steady state). The relationship AUC_0–∞ = AUC_0–τ holds for linear PK (superposition principle).

❝ I believe I cannot subtract baseline value from all the samples as commonly done in case of endogenously produced molecules and therefore all the listed parameters except t1/2 may not accurately represent drug profile.

Correct.

Example: Five minutes infusion, dose 2× 100 mg, τ four hours. I simulated data in such a way that C_max in the second profile is 200 µg/mL and C_τ is 10 µg/mL.

 t        C  
0.0000   0.00
0.0833 191.04
0.2500 168.17
0.5000 138.90
0.7500 114.73
1.0000  94.76
1.2500  78.27
1.5000  64.64
2.0000  44.10
3.0000  20.52
4.0000   9.55
4.0833 200.00
4.2500 176.06
4.5000 145.42
4.7500 120.11
5.0000  99.21
5.2500  81.94
5.5000  67.68
6.0000  46.17
7.0000  21.49
8.0000  10.00

Note that the first C_max is <200 µg/mL. That’s as expected since there is some accumulation. Also the second pre-dose concentration is only 9.55 µg/mL.
In WinNonlin you should define Dose Options | Type IV Infusion and Calculation Method Linear Up Log Down. For the second profile Setup | Dosing ☑ Use Internal Worksheet.

Dose │ Time of Dose │ Length of Infusion │ Tau
(mg) │              │                    │    
─────┼──────────────┼────────────────────┼────
 100 │            4 │             0.0833 │   4

It is important to give Tau 4. If you leave the cell empty (the default) WinNonlin will “assume” that the profile came from a single dose and steady state PK metrics will not be calculated.
You should get:

Cmax        200      µg/mL
Cmin          9.55   µg/mL
Clast        10      µg/mL
Cavg         64.286  µg/mL
Fluctuation 296.25   %
AUC_TAU     257.15   h*µg/mL
Vz          508.44   mL
Vss         508.98   mL
Acc_Index     1.0492

Note also that AUC_TAU is calculated for the specified τ (by inter-/extrapolation if the last sampling time point deviates from τ). That’s important since in such a case AUC_TAU ≠ AUClast.