Antibiotics 2.

Imipenem cilastatin is the next product we will talk about. This is Primaxin. Hopefully you don’t have to use this drug too often but when you do you may find it to be quite useful. Basically imipenem is metabolized by the kidney. Cilastatin cuts down on its metabolism. It’s useful in Gram negative bacilli, anaerobes and Staph resistant to other antibiotics. Seizures can be a problem with this product and it is very expensive. It can induce resistance to all other penicillins and cephalosporins so if you’ve made a choice to put your patient on imipenem cilastatin or Primaxin, you DC any cephalosporins and any penicillins thereon because the patient will become resistant to them.
Meropenem is a newer product that is marketed as Merrem. It is like imipenem but it doesn’t require the cilastatin and it does have an increased incidence of seizure problems. It is used essentially the same as would be imipenem and it is a very expensive product. But in certain circumstances like severe resistant Gram negative infections, you may be backed into a corner and end up having to use something like this.
I use the term monobactams here but there really is only one monobactam currently marketed. This is a single ring from the basic penicillin nucleus. Rather than having two rings it has only one of them so there is some risk for cross allergenicity with penicillins and cephalosporins but it is reasonably low. Again, probably be cautious if they have a rash history and use it with extreme caution if there is an anaphylactoid history of penicillin or cephalosporin allergy.
This is useful strictly in Gram negative aerobes. It has no effect against Gram positive or anaerobes. Aztreonam is the drug that is a monobactam. This may have some synergy with aminoglycosides versus Pseudomonas and may be useful in certain, for example, abdominal uses for prophylaxis or treatment. The problem here, though, is because it does not cover Gram positives, you may suppress the Gram negatives and get Gram positive super infection.
Switching away then from the beta-lactam drugs we now come to the macrolide antibiotics. Erythromycin is the one that we’ve had around forever. It can be used as a substitute for penicillin. However, pneumococcal and streptococcal resistance is a growing problem with this particular drug. You no doubt are aware that it can be used for lots of other things like Campylobacter and mycoplasma. It is a drug that can be used for impetigo although about one-third of patients will get pretty severe GI upset when you are giving oral erythromycin. Many clinicians have switched to using Keflex or cephalexin for systemic treatment of widespread impetigo and that appears from the literature to be equally effective with erythromycin and probably a lot better tolerated. So for widespread impetigo that might be a reasonable choice.
Toxicities with erythromycin. GI upset is a major one. Remember it does stimulate motilin activity in the gut. It causes GI hypermotility. We use erythromycin in some patients to stimulate GI motility but it is that same stimulation that causes a lot of the GI upset. So giving it with food (some people will try it to cut down the GI upset) often does not have much of an effect because you get the same release of motilin. More importantly these drugs can potentially cause hepatotoxicity. The drug that is most often associated with it is erythromycin estolate or Ilosone although erythromycin ethylsuccinate can sometimes cause it. Erythromycin ethylsuccinate is marketed as numerous products but it probably is less likely to cause it and many clinicians suggest that you try E.E.S. If they get significant GI upset and you still want to stay with an erythromycin then try erythromycin estolate or Ilosone. Others will say, “Well, we’re going to use erythromycin estolate first in children because it is the best absorbed and best tolerated but will avoid using it in adults.” That’s probably a wise suggestion. The use of Ilosone should probably be avoid in adults because the risk of hepatotoxicity tends to increase with age.
Erythromycin derivatives do also come IV but the IV is extremely irritating. It’s almost impractical to use this drugarenterally. There is no IM erythromycin product available.

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