MYLOTARG® (gemtuzumab ozogamicin), also referred to as CMA-676 or just CMA, consists of a monoclonal antibody against CD33 that is bound to calicheamicin by means of an acid-hydrolyzable linker. When the derivatized calicheamicin binds to the DNA minor groove, it disrupts DNA progression and eventually causes cancer cell death. The commercial product was marketed as the first antibody-targeted chemotherapeutic agent under the name MYLOTARG® and is currently approved for the treatment of acute myeloid leukemia (AML) in elderly patients.
3-Methyl-3-mercaptobutanoic acid hydrazide, also called DMH linker, or CL-332258, is used to link calicheamicin to monoclonal antibodies. The derivatized calicheamicin is then activated for conjugation with a humanized monoclonal antibody to give CMA-676. Currently, DMH linker is prepared through a 5-step reaction process via the intermediate, p-methoxybenzylthioether hydrazide, 5. (Equations I-V). In the present manufacturing process, a Michael addition of p-methoxy-benzylthiol to 3,3 dimethylacrylic acid is assisted by piperidine, (Equation I).

The resulting thioether acid (1) reacts with oxalyl chloride in methylene chloride to form p-methoxybenzylthioether acid chloride (2) (Equation II).
Acid chloride (2) is slowly added to a mixture of hydrazine/methylene chloride (in a ratio of about 28%, v/v) at low temperature (−70° C.). The corresponding p-methoxybenzylthioether hydrazide (3) forms in about 74% yield (Equation III):

However, the desired product p-methoxybenzylthioether hydrazide (3) typically contains about 20% of an undesired by-product, bis-methoxybenzylthioether hydrazide (see Equation VI below). Removing the benzyl protecting group under acidic conditions (Equation (IV), followed by neutralization of the acid salt and purification (Equation V) provides 3-methyl-3-mercaptobutanoic acid hydrazide (5) in 45% yield.

An undesired by-product, bis-methoxybenzylthioether hydrazide (6) is generated from the reaction of the product p-methoxybenzylthioether hydrazide with the starting material p-methoxybenzylthioether acid chloride (Equation VI). The generation of this by-product results in lower yield and quality.

Using original process procedures, the bis-methoxybenzylthioether hydrazide (6), is generated in amounts of about 20%. The presence of this level or greater of undesired by-product from Equation III is clearly undesirable. The present invention describes techniques which overcome this problem and decrease the yield of the undesired by-product.