National Cancer Institute drug screening. growth inhibition parameters?? - (May/12/2011 )

Hi All

Can someone please help explain the following calculations carried out by the NCI 60 cell line screen when testing drugs.
They initially plot a percentage growth inhibition curve using the following equations
Percentage growth inhibition <(Ti-Tz)>/C-Tz x 100 when Ti > Tz

Ti = absorbance after drug treatment Tz = absorbance at time zero, C = absorbance of control growth.

When Ti< Tz they dont factor the absorbance of the control growth and just use the equation <(Ti-Tz)>/Tz x 100 to calculate percentage growth inhibition

Can someone explain why the control growth is not considered when Ti < Tz surely if Ti is less than Tz its not cell inhibition but cells killed?

The url is below for full explanation
http://dtp.nci.nih.gov/branches/btb/ivclsp.html
Im lost!!
Any help would be great.

bonchem on Thu May 12 11:58:28 2011 said:

Each compound has 2 parameters calculated. One is inhibition of control cell growth (minus Tz) shown by the first equation. The second is inhibition of the baseline response at the time the compound was added (Tz), time zero. Therefore the second equation is only measuring inhibition of the baseline response and is a measure of net cell killing. You could measure only total inhibition of the control growth like you suggest but then you would get >100% inhibition if the compound inhibited beyond Tz and it's not obvious this is due to an inhibition of Tz (as the percentage of cell kill is likely to be relatively small). It's easier for screening if total growth = 100, total inhibition of growth = 0 and total cell kill = -100.