# Medicine:Cmin

Short description: Minimum blood plasma concentration of a drug during the time between two doses

Cmin is a term used in pharmacokinetics for the minimum blood plasma concentration reached by a drug during a dosing interval, which is the time interval between administration of two doses. This definition is slightly different from Ctrough, the concentration immediately prior to administration of the next dose.[1] Cmin is the opposite of Cmax, the maximum concentration that the drug reaches. Cmin must be above certain thresholds, such as the minimum inhibitory concentration (MIC), to achieve a therapeutic effect.[2]

In most cases Cmin is directly measurable. At steady state the minimum plasma concentration can also be calculated using the following equation:[3]

$\displaystyle{ C_{min}= \frac{SFD k_a}{V_d(k_a-k)}\times\{\frac{e^{-k\tau}}{1-e^{-k\tau}}-\frac{e^{-k_a\tau}}{1-e^{-k_a\tau}}\} }$
$\displaystyle{ S }$= Salt factor
$\displaystyle{ F }$= Bioavailability
$\displaystyle{ D }$= Dose
$\displaystyle{ k_\text{e} }$= Elimination rate constant
$\displaystyle{ k_a }$= Absorption rate constant
$\displaystyle{ V_d }$= Volume of distribution
$\displaystyle{ \tau }$= Dosing interval

Cmin is also an important parameter in bioavailability and bioequivalence studies, it is part of the pharmacokinetic information recommended for submission of investigational new drug applications.[4]

## References

1. Weimann, H. J.; Cawello, W.; Brett, M.; Zimmermann, H.; Pabst, G.; Sierakowski, B.; Giesche, R.; Baumann, A. (1999). "Drug concentrations and directly derived parameters". Parameters for Compartment Free Pharmacokinetics: Standardisation of Study Design, Data Analysis and Reporting. Aachen, Germany: Shaker-Verlag. pp. 25–40. ISBN 9783826547676. OCLC 44511664.  (pages 31–34 in 2003 edition)
2.