Five-week-old female CB-17 severe combined immunodeficient (SCID) mice were obtained and quarantined for 7 days prior to study initiation Mice were inoculated subcutaneously with KB cells (1 x10⁷ cells per mouse) resuspended in serum-free culture mediaMice with established KB xenografts were dosed with a single intravenous injection of 200 g of conjugated maytansinoid per kg, equivalent to 10² mg/kg antibody on day 6 after cell inoculationTumor dimensions were measured twice weekly and volume was calculated as (A B2) 05, where A represents the largest and B the perpendicular tumor diameter.

Dilutions of conjugates or unconjugated maytansinoid in the appropriate culture medium were added to wells of 96-well flat-bottomed plates containing 1 x10³ cells per well The plates were incubated at 37°C, 6% CO₂ for either 5 days (continuos exposure) or for 4 hours followed by 5-day incubation in conjugate-free medium (short exposure). Cell viability was determined by the WST-8 assay in accordance with the manufacturer's protocol, and IC50 were generated using a sigmoidal dose-response (variable slope) nonlinear regression curve fit.