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I have followed your discussion on "once daily" gentamicin
administration with great interest. Sadly, there are also some
serious adverse effects linked to this problem as we have heard.
Having worked a great deal with gentamicin PK lately, I'd like to add
the following to the discussion.
1. Our data on two animal species (dogs and horses) and our
preliminary data in humans indicate that the gentamicin components
C1, C1a and C2 have different pharmacokinetics (CL, Vss and t1/2).
The differences are to some degree species dependent but generally
similar. Therefore, it would be very surprising to me if
administration protocols would be similar to all gentamicin
preparations because of the great variability in the component ratio
among those preparations. Even the USP method to determine the
component ratio appears biased. To my understanding also the amount
of total gentamicin in them varies because they a standardized
according to their antimicrobial activity and not the amount of drug
on weight basis (please, correct if I am wrong). Therefore, I
completely agree with Roger Jelliffe with the need for individual TDM
(and the understanding of the equations). However, the fluorescent
polarization immunoassay, that appears to be the standard, does not
differentiate between the compounds and, consequently, produces an
2. Does anybody of the group know anything about the different
toxicity of the gentamicin components beyond the published
literature? In my understanding there was an obvious effort to
produce a single component gentamicin preparation in the 70's but it
faded away for reasons that were not documented.
3. Does anybody have any information on the antimicrobial potency
against clinical strains of the different gentamicin components?
4. Does anybody know of a source to obtain certified reference
material or reference material of the gentamicin C1, C1a and C2
I don't expect to solve all the problems involved by gentamicin
component ratio but I am also convinced that a lot of the
uncertainties about gentamicin results from the fact that we do not
really always talk about the same thing (drug).
With best regards,
Stefan Soback, DVM, PhD
Kimron Veterinary Institute
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I'm also interested if anyone has ever seen any work or had any human
clinical experience with different toxicities associated with the
stereoisomer of gentamicin used. I once knew a physician who claimed to
have done work showing Schering's Garamycin was less toxic than the
generics on the American market due to the presence of different chiral
components between the two products.
Richard Molitor, R.Ph.
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