Fundamentals II: 10:00-11:00am Scribe: Susanna Pischek
Wednesday, December 9, 2009 Proof: Ashley Brewington
Dr. Jeff Smith Chemotherapy Page 6 of 6
I. Antibiotics [S18]
a. I will continue with our discussion of antibacterials, and I would like to attempt to cover quite a number of antibacterials in a short order today.
b. They are classified according to their target of action.
c. So I started to sketch out on the board where we’re going today.
d. We’re going to start with cell wall synthesis inhibitors, which we talked some about in terms of mechanisms of actions yesterday. We’ll talk more about especially the classes of penicillins and cephalosporins. We’ll give some attention to another cell wall synthesis inhibitor that I mentioned yesterday is increasingly important for treating infections caused by methicillin resistant staph aureus (MRSA). That’s the antibiotic vancomycin.
e. Then we’ll leave the cell wall synthesis inhibitors and move to the antibacterials that act on the bacterial ribosome that inhibits protein synthesis in bacteria. We’ll begin with discussion of two groups of antibacterials that are namely aminoglycosides, such as streptomycin. And then the other class of antibacterials that act on the smaller of the two subunits of the bacterial ribosome are the tetracyclines. So, we’ll talk aminoglycosides, tetrcyclines, and then we’ll move to the antibacterials that act on the larger subunit of the bacterial ribosome.
f. So the bacterial ribosome of course usually is drawn as two circles with two subunits. The 30S and the 50S. Of course it translates the mRNA for various bacterial genes and produces a protein from each gene. So we’ll talk then about the macrolides, such as azithromycin and clindamycin which is primarily active against gram positives and some anaerobes.
g. And I may not have time to say much about chloramphenicol, CAP. This is an example of a broad spectrum antibacterial true antibiotic. It is produced by a soil bacteria, active against gram positives, gram negatives. Also unusual bacteria. Its use is very limited as I mentioned yesterday because it produces a rare fatal toxicity, which it is due to the complete loss of all blood cells. So, it causes what is known as aplastic anemia. So, therefore, it is not used much in the US although it’s still widely used in other countries.
h. Then, the next group of antibacterials are the DNA gyrase inhibitors. And that’s the principally the fluoroquinolones. I probably won’t have too much time to say hardly anything about them. They are active against gram negatives and also gram positives. They are used for treating a variety of infections. They are covered in your handout.
i. Then a couple of others are membrane active polymixines, which are too toxic to be used internally but they’re used topically to treat skin infections, for example. They are chiefly active against gram negatives. They are covered in the handout.
j. I am calling your attention to some that I won’t have time to cover, which include the last one which is metronidazole. Metranidazole was initially used for treating protozoa infections, and then it was subsequently found to be quite effective against anaerobes. Anaerobic bacteria, bacteria that grow under anaerobic conditions, lack of oxygen. They are active against anaerobes, both gram positives and gram negatives. I call your attention to the common feature of metranidazole.
k. Common structural element of metranidazole and chloramphenicol, which is unusual. It is that they have a nitrogen and two oxygens. It is called a nitro group. So chloramphenicol has a nitro group as a part of its structure, as does metronidazole. That nitro group is reduced by bacterial enzymes. It becomes reactive with DNA. So, these two antibacterials, metranidazole and chloramphenicol, both have a nitro group as part of their structure. Can cause DNA damage. It’s how metranidazole is able to kill anaerobic bacteria because anaerobic bacteria are especially capable of reducing that nitro group. And that is how metranidizole works. It is reduced and becomes highly reactive with DNA, so it damages DNA. But there’s a risk associated with that of course. It is that our DNA is structurally, chemically the same as bacterial DNA. We have the same bases, purines and pyrimidines in our DNA as bacteria. So, this nitro group will cause mutations in our DNA, and metranidazole does cause cancer in experimental animals. There is some risk associated with antibacterials that have a nitro group. It’s not known whether the nitro group of chloramphenicol is responsible for the loss of all blood cells, the aplastic anemia, that’s caused by in a rare incidence by chloramphenicol. But certainly people are suspicious of that nitro group as being the causative factor. It is probably more complex because it involves in the case of metranidazole, the anaerobic bacteria. The reason metranidazole is active against anaerobes is because anaerobes are the class of bacteria that are able to reduce the nitro group.
l. So, I think we left off with this diagram here, and I am going to move very quickly through this MIC, MBC. They are useful terms for quantifying the potency of antibacterials.
m. So, antibacterials that only inhibit the growth of bacteria are characterized by an MIC, which is the minimal concentration of the antibacterial that is sufficient to stop the growth of the bacteria, say in a 24 hour interval.
n. The MBC is the minimal bactericidal concentration. So that would only apply to bactericidal antibacterials. That’s the minimal concentration of the antibacterial that is able to kill 99.9% of a population of bacteria in say a 24 hour period.
o. The more we understand the mechanism of action of the antibacterials, the better we are at efficiently using them to eradicate an infectious agent without producing a toxic effect in the patient.
p. So, for example, the beta-lactams are characterized by a time dependant killing. So the degree of killing of a population of bacteria sensitive say to penicillin depends on the duration of the exposure of the population of bacteria to the penicillin not on the concentration above the MBC. And then in contrast there is also concentration dependant killing, which is exemplified by antibiotics such as streptomycin or tobramycin, gentamycin and the aminoglycosides. This concentration dependent killing means that the killing effect is proportional to the concentration of the aminoglycoside above the minimal bactericidal concentration.
q. Aminoglycosides also have a substantial post-antibiotic effect, which means that they are able to continue killing. And aminoglycosides are bactericidal of course. So they have a capacity to kill even when the concentration of the aminoglycoside falls below the minimal bactericidal concentration.
r. The reason that is noteworthy, the reason we’re getting into kind of esoteric details concerning the mechanism and the description of the antibiotic action of these different antibacterials, the reason it is so important with aminoglycosides the realization that they have this post-antibiotic effect, the reason it’s important is because of course aminoglycosides are quite toxic. They are given usually IV, hospitalized patients. They can cause irreversible damage to the kidneys, also to the inner ear. So, they cause disturbances in balance. They can cause deafness, loss of hearing.
s. By taking advantage of the post-antibiotic effect of the aminoglycosides, it is possible to prolong the interval between doses. Of course the toxic effects of the aminoglycosides, kidney damage, loss of hearing, are dependant on the duration of exposure to the aminoglycosides. So by taking advantage of the post-antibiotic effect, you can prolong the interval between dosing and minimize the chance of irreversible kidney damage or damage to the ear. So, that’s the significance of these terms.
II. Resistance to Antibiotics [S19]
a. I’m going to move on now. Dr. Waites discussed antibiotic resistance with you. So, I cover it in the next few slides, but I’m not going to talk about them because Dr. Waites covered it.
b. But I just indicate in these next diagrams, I want you to be aware they are in the handout, that there are three major types of resistance.
c. Resistance due to inactivation, chemical inactivation by bacterial enzymes inactivate the antibacterial or decreased accumulation of the drug by the bacteria, or lastly the alteration of the drug target.
III. Resistance to Antibiotics I [S20]
a. And then the next slides give examples of antibacterials that are inactivated by enzymes or are expelled from bacteria like tetracylines. And then lastly alterations in drug target.
IV. Resistance to Antibiotics II [S21]
a. Skipped
V. Resistance to Antibiotics III [S22]
a. Skipped
VI. Beta-lactam antibacterials [S23]
a. So, now we can begin this survey of the five major classes of anitbacterials with a couple of additional agents thrown in. But we’ll see how far we can get with this.
b. I want to spend most of the time on the discussion of the cell wall synthesis inhibitors and the aminoglycosides and tetracyclines. Then we may carry on with this after class. I hope you find the handout to be a clear, concise summary of the key features of these major antibactials that we may not have time to cover in class.
c. So, beta-lactams are the penicillins and cephalosporins. They are called beta-lactams because they have a beta-lactam ring as a central component of bacterial activity. The beta-lactam ring is a four membered ring, which is composed of three carbon atoms and a nitrogen atom. There’s also an oxygen atom coming off the beta-lactam ring. Then there’s additional parts of the molecule to complete the anti-bacterial.
d. But the beta-lactam ring, as I say, is the essential component. The reason it is essential, as we talked about yesterday, it’s the acylating portion of the penicillins and cephalosporins. So, the beta-lactam ring is the chemically reactive part of all the beta-lactams, the penicillins and cephalosporins.
e. One other important beta-lactam that I haven’t mentioned up to this point—can anyone help me with what I’m thinking of? It’s not a penicillin; it’s not a cephalosporin. But it has a beta-lactam ring. Anybody? What is it? It has a beta-lactam ring, but it’s not a penicillin or cephalosporin. We’re going to talk about it in a minute. It doesn’t work by inhibiting the cell wall. What does it do? I’m talking about a class of agents that are used in combination with penicillins to extend the antibacterial spectrum to bacteria that destroy the beta-lactam ring, right?
f. So, we’re talking about bacteria that have penicillinase or it’s also called beta-lactamase. Those bacteria are resistant to the original penicillins and to most penicillins except the class II pencillins because they destroy the beta-lactam ring. They hydrolyze it. At that point they add water across that bond and break open the beta-lactam ring, destroy the anti-bacterial activity of the penicillins. As I say, they are bacteria that have the enzyme that have the enzyme beta-lactamase or penicillinase.
g. So, what the group of drugs that I’m referring to then are the inhibitors of beta-lactamase. They have a beta-lactam ring. Although they have a beta-lactam ring, they’re not sufficiently potent to be useful as an anti-bacterial by itself. Why? Because they don’t react with the PBPs.
h. Okay, we’re talking about beta-lactamase inhibitors, which are used in combination with some of the penicillins such as amoxicillin. So, they also have a beta-lactam ring. And the beta-lactam ring is essential for inhibiting beta-lactamse. But they are not sufficiently potent to be able to react and inactivate a PBP, such as transpeptidase. So, that is the beta-lactamase inhibitors exemplified for example by clavulanate. That’s a commonly used beta-lactamase inhibitor.
VII. Beta-lactams [S24]
a. Let’s talk about the major classes of penicillins.
b. This diagram just shows the beta-lactam ring that I drew on the board.
VIII. Penicillin V potassium[S25]
a. So, I’m going to move ahead and begin with just calling attention to this Penicillin V, which is a narrow spectrum penicillin, chiefly active against gram-positive bacteria.
b. The first class of penicillins includes penicillin V, shown here, and also penicillin G, the original penicillin. Pen V, like Pen G, has an ionized carboxyl group. So, it is administered as a salt. The preferred salt is the Potassium salt. So, the potassium ion balances the negative charge of the carboxyl group. The salt form of the penicillin G or V is what’s administered usually orally, or also IV.
IX. Five Major Classes of Penicillins [S26]
a. Here’s the classes of penicillins. Five major classes. The first class, as I’ve said, are the original ones.
b. They are destroyed by penicillinase, or beta-lactamase. Yet they are still widely used because they’re among the most potent of all antibacterials for treating infections caused by certain gram positive bacteria. So, they’re still very important, those original penicillins. They are narrow spectrum, just gram postitives.
c. And then the second class are highly important and distinguishable from the other four classes because they are active against penicillinase resistant bacteria. In other words, these second class penicillins, such as methicillin, oxacillin, cloxacillin, dicloxacillin, these are relatively resistant to penicillinase, unlike class I or class III, IV, and V. I should add one point about this second class, which is they are also chiefly active against gram positives. But gram positives that produce penicillinase, okay.
d. Then with class III, IV, and V have activity against gram negatives but tend to have less activity against gram positives. Although class III are still active against some gram positives. Class IV and V mostly are rather weak with poor activity against gram-positives but increased activity against gram negative bacteria.
e. So, as you go down the classes, you get increased activity against gram negatives and decreased activity against gram positives. Those are the five major classes of penicillins.
X. Classes of Beta-lactamases [S27]
a. Let’s briefly mention a little more about the Beta-lactamase inhibitors. So, I want to call your attention to the fact that there’s lots and lots of different beta-lactamases produced by different bacteria. They differ by these four criteria listed in this diagram.