Pharmacokinetic analysis of TH1902After the linkers, my other special point of interest is the pharmacokinetic study of TH1902. I repeated many times here that it is very important to understand that at this point Marsolais et al at Thera have a pretty good idea of the fate of TH1902 in humans after injection because blood samples are taken at many different time points and analyzed for TH1902 content and free docetaxel content. In the recent article, they give a good description of how they proceed to do that in mice, and it is likely to be very similar on humans. That allows them to compare the behavior of the drug between patients and correlate that with their level of sortilin expression on tumors, and also the tumor load (the higher the tumor load, the more sortilin you will have for a given level of expression). It is not a very precise correlation, but by comparing relatively extreme cases it can allow some indirect understanding of TH1902 internalization by cancer cells expressing sortilin. If they compare the data of a patient with no sortilin expression on the tumors, and another one with high expression and a relatively high tumor load, the pharmacokinetic data of TH1902 between the two should not be similar. They should see lower levels of TH1902 and free docetaxel in the blood over time for the patient with sortilin expression. In the mice the blood samples were taken at 5, 15, 30, 60, 120, 240, 360 minutes after injection. Given that the half-life of TH1902, according to Marsolais, is around one hour, this is long enough to understand what is going on.
Now that we know the sequence of TH1902 I calculated the molecular mass of the drug.
TH19P01 + succinyl + succinyl + docetaxel + docetaxel = TH1902
1922 +84 + 84 + 807 +807 = 3704
That allows to clearly understand why they say 230 mg/m2 of TH1902 equals 100 mg/m2 of docetaxel.
Given that each molecule of TH1902 contains two molecules of docetaxel and that the molecular weight of docetaxel is 807.
3704/2 = 1852 x 100/807 = 230
2.15 Pharmacokinetic analysis
Plasma samples for pharmacokinetic characterization were collected at 5, 15, and 30 minutes as well as at 1, 2, 4, and 6 hours following i.v. bolus injection of TH1902 into normal CD-1 mice with lithium heparin microvette tubes (Sarstedt). TH1902 and docetaxel concentrations were determined by UPLC/MS. Briefly, both TH1902 and docetaxel were extracted from 50 µL of plasma using 4 volumes of cold acetonitrile (87%):acetic acid (0.125%):water (12.875%) in micro centrifuge tubes. After 5 minutes on ice, the suspensions were centrifuged at 10 000 g for 5 minutes and supernatants (2-10 µL) were injected into a Waters Acquity UPLC spectrometer coupled to a microTOF spectrometer from Bruker using electron spray ionization (ESI-TOF). TH1902 and released docetaxel from the peptide were quantified by peak area ratio at a wavelength of 220 nm. Standard curves for TH1902 and docetaxel were performed in mouse plasma. A linear regression was carried out to determine the concentration of TH1902 and docetaxel. The limit of quantitation for TH1902 and for free docetaxel was around 8 µM.