Problem:

Magic bullets are drugs that can be administered on a micro local scale. In this context administration/binding would occur in or near the tumour by exploiting the different surface antigens that cancers expose. Basic lectures teach that a "magic bullt" drug could attach via an mAb (attached to the pro-drug) and then a conjugated enzyme cleaves or converts the pro drug to a drug. This would eliminate a lot of side effects from chemotherapy.

Use in clinical trials. Some mAbs have started to be used in clinical trials, almost 40 years after they were first proposed.

Question: Have there been any cases of mAbs being used as a successful treatment for any disease? If not, what makes the R&D of this drug type so difficult?

Please explain in detail.