## Reinventing the Hula-Hoop [NCA / SHAM]

Dear Friends!

I don't quite keep up with all directions of thought, but I am extremely grateful for your profound answers! I will discover your explanations and links step by step.

Dear Helmut!
You adviced me not to google VRT, but how could I avoid this temptation?
Now I think that you are slightly confused: VRT do not define the center of gravity. According to J.E. Riviere1:

$$VRT=[\int (t-MRT)^2\cdot C(t)dt]/AUC$$.

It has t2 under the integration, so it is just what we consider as an analogue of moment of inertia, that is the second moment, besides its dimension is 'time2'.

VRT is used to calculate the coefficient of variation of residence times (CVRT):

$$CVRT=\sqrt (VRT)/MRT$$,

which is a dimensionless parameter that provides 'the dynamics and heterogeneity of drug distribution'.

On the other hand the center of gravity would be defined by the following equation:

$$C_c=1/2\cdot[\int C(t)^2\cdot dt]/AUC$$.

It has the dimension of concentration. It was suggested by Lassen and Perl in 1979 to use it as a perfect parameter that is sensitive to both the rate and the extent of absorption, which is not the case for the AUMC/AUC measure.

That same feeling when you invent something that was already invented long before you was born (considering the center of gravity of hula-hoop - links to the concept of solipsism)

» Only nice to print a profile, cut it a out, push a pin through it, and make a weird whizz wheel for kids.

You've already discussed it here (thanks to mittyri I've finally found that post and finally got rid of deja vu).
Obviously as you've mentioned C2 will give us much variability, but may be it is not worth to throw momentally this parameter to the bin. It reflects the extent of absorbtion and could reflect the major properties of the curve even in the case when it is outside it. Not the case of MDT. Returning to my initial question: what do you think about calculating MDT for multiphase profiles?

Returning to MRT: two similar parameters should be mentioned:
MAT Mean Absorbtion Time (MAT=MRTni-MRTIV, where ni is any noninstanteneous administration) and
MTT Mean Transit Time2.
[1] Riviere, J. E. Comparative Pharmacokinetics Principles, Techniques, and Applications, DOI:10.1002/9780470959916
[2] Veng - Pedersen, P. 1989a. Mean time parameters in pharmacokinetics: defi nition, computation and clinical implications (part I). Clinical Pharmacokinetics. 17 : 345 – 366.
[3] Veng - Pedersen, P. 1989b. Mean time parameters in pharmacokinetics: defi nition, computation and clinical implications (part II). Clinical Pharmacokinetics. 17 : 424 – 440.

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