Thursday, March 18, 2004

Multidrug-resistant TB

Lessons About Medical Reasoning
The World is Full of Surprises

On March 16, 2004, the World Health Organization released a report on multidrug-resistant tuberculosis.  This is a serious problem, especially in the former Soviet states of eastern Europe.  This is an example of a trend in world health: more and more often, we are seeing infections caused by bacteria that are able to resist treatment with antibiotics.  A lot has been written about the topic, so I will not discuss the more fundamental aspects of the problem.  See this article  for a good background review of antibiotic resistance, and this site for basic information about TB.  Derek Lowe on Corante  has a detailed explanation.  Blogger comments are here: 1 2 3

I often hear people say that infectious disease is one of the more straightforward kinds of illness to understand and to treat.  Well, simplicity is relative.  Compared to something like rheumatoid arthritis, most infectious disease are simple.  But this apparent simplicity is deceptive.  Take the example of tuberculosis.  TB is caused by a kind of bacterium, Mycobacterium tuberculosis.  People are not supposed to have  these bacteria growing in them.  When they do, they are sick.  The sickness is treated by giving a chemical that kills the bacteria.  Seems simple.  It turns out, though, that the battle between us and TB has been going on for some time.  Both adversaries have developed sophisticated means to try to outwit the other.  The whole topic is incredibly complex, but to illustrate, I cite the following article:

Researchers Find How Tuberculosis Bacterium Evades Detection By Immune System

A new study published in the July 15 issue of the "Journal of Immunology" may unlock a door in the search for a vaccine. The study from Case Western Reserve University's School of Medicine and University Hospitals of Cleveland details how the tuberculosis bacterium evades detection by the body's immune system.

[...] When an infection invades the body, the immune system is called upon to control and stop the infection. Important soldiers in the war against infection are scavenger cells called macrophages which chew up invading bacteria and deliver pieces of them to white blood cells named CD4 T cells.

Macrophages have a specialized set of molecules, called MHC-II (which stands for class II major histocompatibility complex). This set of molecules is used to present the pieces of invading bacteria to CD4 cells. These pieces, called antigens, are the way CD4 cells can recognize and eliminate invading bacteria.

The CWRU/UHC researchers have discovered that the TB bacterium stops the immune system from using this important piece of equipment from its arsenal. The bacterium inhibits the specialized MHC-II molecules by taking up residence in the macrophages and making a large protein in abundant quantities which interferes with MHC-II production.

Furthermore, the bacterium does this while engaging a macrophage receptor normally used for protection against a large number of infectious diseases. By employing that receptor and inhibiting MHC-II molecules, the bacterium evades detection.

Even before the introduction of antibiotics, TB evolved a mechanism to evade detection by the human immune system.  As soon as antibiotics were introduced, TB began evolving in ways that cause the antibiotics to become less effective.  Naturally, this is an alarming development, one that poses a problem begging for a solution. 

To attempt to solve this problem, the first temptation is the Tool Time Method: "more power."  That is, develop a second antibiotic that is stronger that the first one.  Generally, this is done by generating a zillion new molecular entities and testing them, one by one, to see how effectively they kill the bacteria.  The problem with this approach is that it, too, will fail eventually.  The bacteria can evolve faster than we can develop new drugs, at least using the trial-and-error approach.  Thus, the Tool Time approach will, at best, buy us a little more time:

Drug resistant tuberculosis levels ten times higher in Eastern Europe and Central Asia
WHO Global Report

16 MARCH 2004 | GENEVA -- Tuberculosis patients in parts of Eastern Europe and Central Asia are 10 times more likely to have multidrug-resistant TB (MDR-TB) than in the rest of the world, according to a World Health Organization (WHO) report into the deadly infectious disease. China, Ecuador, Israel and South Africa are also identified as key areas.

[...] MDR-TB is TB that is resistant to the two medicines most commonly used to treat it, Isoniazid and Rifampicin. Without the correct drugs MDR-TB is untreatable and in most cases fatal. Though curing 'normal' TB is cheap and effective - a six month course of medicines costs US$ 10 - treating drug resistant TB is a hundred times more expensive. Even then a cure is not guaranteed. With no effective vaccine, everyone is vulnerable to infection simply by breathing in a droplet carrying a virulent drug resistant strain.

[...] WHO's leading infectious disease experts estimate there are 300 000 new cases per year of MDR-TB worldwide. There is also new evidence proving drug resistant strains are becoming more resistant, and unresponsive to current treatments. 79% of MDR-TB cases are now "super strains", resistant to at least three of the four main drugs used to cure TB.

The second strategy is to figure out how the bacteria are overcoming our first strategy.  That involves studying, on a molecular level,  how the bacteria become resistant to antibiotics.  This takes a bit of effort, but it is possible to do it because the bacteria are simple structurally, compared to humans:

Multidrug-Resistant Mycobacterium tuberculosis: Molecular Perspectives
Emerging Infectious diseases, Vol 4, No. 2 April-June 1998

Ashok Rattan, Awdhesh Kalia, and Nishat Ahmad
All India Institute of Medical Sciences, Ansari Nagar, New Delhi, India

Multidrug-resistant strains of Mycobacterium tuberculosis seriously threaten tuberculosis (TB) control and prevention efforts. Molecular studies of the mechanism of action of antitubercular drugs have elucidated the genetic basis of drug resistance in M. tuberculosis. Drug resistance in M. tuberculosis is attributed primarily to the accumulation of mutations in the drug target genes; these mutations lead either to an altered target (e.g., RNA polymerase and catalase-peroxidase in rifampicin and isoniazid resistance, respectively) or to a change in titration of the drug (e.g., InhA in isoniazid resistance). Development of specific mechanism–based inhibitors and techniques to rapidly detect multidrug resistance will require further studies addressing the drug and drug-target interaction.

Understanding the molecular basis of drug resistance can help us redesign old drugs, develop new drugs, and figure out how to combine two or more drugs for a greater effect.  By increasing the rate of development of new strategies, it might be possible for us to keep pace with the evolution of the bacteria.  This is an example of the problem-solving strategy of first understanding why the problem exists, before trying to solve the problem. 

Ok, so much for linear thinking.  Sometimes, luck turns out to be what does the trick. 

Paroxetine (Paxil) is a chemical, called a selective serotonin reuptake inhibitor,  that is used as an antidepressant.  It slows down the action of a protein on certain nerve cells.  The protein it acts on is the serotonin transporter.  Wouldn't it be really strange if a drug that is used to treat a mental illness turned out to be useful for treating an infectious disease?

Phenylpiperidine selective serotonin reuptake inhibitors interfere with multidrug efflux pump activity in Staphylococcus aureus.
Int J Antimicrob Agents. 2003 Sep;22(3):254-61.

Kaatz GW, Moudgal VV, Seo SM, Hansen JB, Kristiansen JE.

Division of Infectious Diseases, The John D. Dingell Department of Veterans Affairs Medical Center and the Department of Internal Medicine, Wayne State University School of Medicine, B4333, 4646 John R, Detroit, MI 48201, USA. gkaatz@juno.com

Structural variants of phenylpiperidine selective serotonin reuptake inhibitors (P-SSRIs) inhibited the function of two unique Staphylococcus aureus multidrug efflux pumps. The most active compound was the paroxetine isomer NNC 20-7052, which had an IC(50) for ethidium, acriflavine, and pyronin Y efflux of 9, 53, and 18% of its MIC, respectively, against the NorA pump. The unbalanced effect of NNC 20-7052 on the efflux of different substrates suggests the possibility that P-SSRIs function by a physical interaction with NorA. Under the conditions employed pump inhibition partially extended to the resistance-nodulation division (RND) pump AcrAB-TolC, but not to the Pseudomonas aeruginosa RND pumps MexAB-OprM or MexCD-OprJ.

What does this mean?  Just as human nerve cells have proteins that move serotonin across the cell membrane, bacteria have proteins that move antibiotics across the cell membrane.  In the case of bacteria, the pump only goes one way: it removes antibiotics from the inside of the cell, rendering them useless.  These proteins are called efflux pumps.  And it turns out that the efflux pumps can be affected by the same kinds of chemicals that affect serotonin transporters.

High-resolution structures of a multidrug efflux transporter responsible for bacterial resistance. | By Andrea Rinaldi

Bacterial resistance to multiple antibiotics and other drugs is a major, increasingly common problem throughout the world. Resistance is often associated with the overproduction of bacterial inner membrane proteins that are capable of extruding a variety of structurally unrelated drugs, antibiotics, and toxic compounds. In the May 9 Science, Edward Yu and colleagues at the University of California, Berkeley, disclose the structural basis of the activity of AcrAB, a constitutively expressed, major multidrug efflux system of Escherichia coli energized by proton motive force (Science, 300:976-980, May 9, 2003).

Now, it turns out that the mutations that cause antibiotic resistance in TB are not mutations that alter the structure of efflux pumps.  Using a linear problem-solving approach, the tendency would be to focus on the structures that mutate in order to produce drug resistance.  Now, at this point, there is no evidence that paroxetine actually has any clinical utility in the treatment of TB.  Do not rush out and get Paxil if you have TB.  However, the serendipitous finding that paroxetine can slow down the action of the efflux pumps could lead to a new class of drugs that, by themselves, do not kill bacteria; but which, in combination with traditional antibiotics, could render them more effective. 

In the title, I said that the world is full of surprises.  It turns out that the very first drug known to have antidepressant properties was iproniazid.  Iproniazid happens to be an old antibiotic that was used to treat... tuberculosis!