Cell experiment:

Exponentially growing cells are seeded in 20 cm2 dishes with an optimal cell number for each cell line (20,000 for LoVo cells, 100,000 for HT-29 cells). They are treated 2 days later with increasing concentrations of irinotecan or SN-38 for one cell doubling time (24 h for LoVo cells, 40 h for HT-29 cells). After washing with 0.15 M NaCl, the cells are further grown for two doubling times in normal medium, detached from the support with trypsin-EDTA and counted in a hemocytometer. The IC50 values are then estimated as the drug concentrations responsible for 50% growth inhibition as compared with cells incubated without drug[2].

Animal experiment:

Irinotecan has been administered by intratumoral injection at 0.1 cc volume of the appropriate solution, for a doses of 5 mg/kg daily for 5 days, on two consecutive weeks, followed by a 7-days rest period, referred to as one cycle of therapy. Rats receive three cycles over a period of 8 weeks. Control animals receive 0.1 cc of sterile 0.9% sodium chloride solution by intratumoral injection in the same rule of administration as that of animals of group II[1].

Irinotecan HCl Trihydrate (CPT-11) is a semisynthetic derivative of camptothecin with IC50 value of 3.4μM in P388 leukemia [1].

CPT-11 is synthesized to avoid toxicity and to improve therapeutic efficacy of camptothecin. It has been approved by FDA for the treatment of patients with metastatic carcinoma of the colon or rectum whose disease has recurred or progressed following 5-fluorouracil-based therapy. CPT-11 is a soluble prodrug of SN-38, it is converted into SN-38 by carboxylesterase enzymes in vivo. SN-38 is a potent inhibitor of topoisomerase I [2].

CPT-11 is found to have efficacy against drug-resistant as well as drug-sensitive tumor cells. It inhibits the growth of the tumors which are VCR- resistant or ADM-resistant. The inhibition can also occur in VCR- and ADM-resistant tumor bearing mice [1].

References:
[1] Tsuruo T, Matsuzaki T, Matsushita M, Saito H, Yokokura T. Antitumor effect of CPT-11, a new derivative of camptothecin, against pleiotropic drug-resistant tumors in vitro and in vivo. Cancer Chemother Pharmacol. 1988;21(1):71-4.
[2] Slatter JG, Schaaf LJ, Sams JP, Feenstra KL, Johnson MG, Bombardt PA, Cathcart KS, Verburg MT, Pearson LK, Compton LD, Miller LL, Baker DS, Pesheck CV, Lord RS 3rd. Pharmacokinetics, metabolism, and excretion of irinotecan (CPT-11) following I.V. infusion of [(14)C]CPT-11 in cancer patients. Drug Metab Dispos. 2000 Apr;28(4):423-33.