How A New Study Shows That Not All Vancomycin Is Created Equal

It is a rare study I read that literally makes me want to stop everything else I am doing and write a blog post about it. On a recent dreary, gray Sunday morning, I was just catching up on some backlogged journal reading when I picked up the August 2010 issue of Antimicrobial Agents and Chemotherapy. I came across a paper out of the University of Antioquia in Medellin, Colombia by Vesga and colleagues entitled “Generic vancomycin products fail in vivo despite being pharmaceutical equivalents of the innovator.”1 (See http://aac.asm.org/cgi/content/abstract/54/8/3271 .)

Regular readers of this blog and the Handbook of Lower Extremity Infections and those who have attended my lectures know that I have not been particularly kind to vancomycin, given the significant failure rates reported in numerous studies. This study might explain why there are so many failures.

Eli Lilly first developed vancomycin after its discovery as a fermentation product of Amycolatopsis orientalis found in a sample of dirt sent to Lilly from Borneo in 1955. Early formulations had significant impurities that, along with lending the product a brown color, led to its early moniker “Mississippi Mud” as well as significant adverse events. Lilly eventually found ways to highly purify the product to the relatively safe drug we know today. Apparently, those extra steps were secret until the company sold its patented technology for purifying the product in 2005.

I clearly remember an Eli Lilly sales rep calling on me 20 some years ago when the drug first went generic. He pulled out a vial of the newly generic vancomycin and it was sandy brown in color. He then pulled out a vial of the brand named Vancocin and it was, indeed, lily-white … pun intended. His point was simply, “Doc, which would you rather have going into your patient’s vein?” Of course, hospital formularies being the way they were/are, we really did not have a choice as to which we wanted to use.

Getting back to the Vesga study, the investigators studied three generic versions of vancomycin versus the “innovator” product to determine concentrations and clinical efficacy via numerous techniques both in vitro and in an in vivo mouse model.1 In my opinion, this was an elegantly designed, complex study that used multiple techniques including time-kill curves (TKC), broth microdilution and a neutropenic mouse thigh infection model, along with others, while using different strains of S. aureus. Their findings were fascinating. To quote the abstract directly:

“Vancomycin generics were virtually undistinguishable from the innovator based on concentrations and potency, protein binding, in vitro antibacterial effect determined by minimal inhibitory or bactericidal concentration and TKC, and serum pharmacokinetics. Despite such similarities, all generic products failed in vivo to kill S. aureus while the innovator displayed the expected bactericidal efficacy …” (emphasis added).1

The authors conclude that the current standard of “pharmaceutical equivalence predicting therapeutic equivalence” is not true, at least for vancomycin. They make it clear that they are not claiming that the tested generics were in any way poor quality. They certainly met all of the current regulations governing the necessary equivalence to make it to the market.

This study raises a number of questions. Is this finding of pharmacologic equivalence not equaling clinical efficacy unique to vancomycin or could it be a factor in other generic antibiotics? Could the universal use of generic vancomycin in hospital pharmacies account for the increasing clinical failures being seen not only in clinical trials but everyday practice?

Reference

1. Vesga O, Agudelo M, Salazar BE, Rodriguez CA, Zuluaga AF. Generic vancomycin products fail in vivo despite being pharmaceutical equivalents of the innovator. Antimicrob Agents Chemother 2010 Aug; 54(8):3271-9.

Editor’s note: This blog originally appeared at http://www.leinfections.com/2010/08/ and has been adapted with permission from Warren Joseph, DPM, FIDSA, and Data Trace Publishing Company. For more information about the Handbook of Lower Extremity Infections, visit www.leinfections.com/



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