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Maitansine (INN), or maytansine (USAN), is a cytotoxic agent. It inhibits the assembly of microtubules by binding to tubulin at the rhizoxin binding site, with a KD of approximately 1 μmol/L. This property made Maitansine a potential anti-cancer drug, with an IC50 of 1.6 x 10-11 M in vivo, approximately 100 to 1000 times more toxic than other anti-cancer drugs. This high toxicity and its lack of specificity resulted in unsuccessful clinical trials.

It is an antibiotic macrolide of the ansamycin type and can be isolated from plants of the genus Maytenus. It is believed to first have been isolated from either Maytenus serrata or Maytenus ovatus.

Maytansinoids
Derivatives of maitansine are known as maytansinoids formed by changing the R-group, shown in red in the above image, to increase target specificity. Target specificity is improved by conjugating the maitansinoids to antibodies.

These derivatives also do not inhibit microtubule assembly as well as Maytansine, but have been seen to suppress dynamic instability, the stochastic switches between growing and shortening, more strongly. Some are being investigated as the cytotoxic component of antibody-drug conjugates for cancer treatment, and the antibody-drug conjugate trastuzumab emtansine is an approved drug for the treatment of certain kinds of breast cancer in the EU and in the US.

Examples of maytansinoids are:
 * Ansamitocin
 * Mertansine / emtansine (DM1)
 * Ravtansine / soravtansine (DM4)