Protease Inhibitors May Have Unexpected Benefits
Compounds become useful drugs only if their beneficial qualities far outweigh any harmful or unwanted activity. But drugs can have subtle, unintended effects -- both good and bad -- that may remain unnoticed until after they are in widespread use. The class of drugs known as protease inhibitors (PIs) caused a revolution in treating HIV after they were introduced in 1996, but they have also been blamed for a range of disturbing metabolic side effects. Now there are reports that some of the HIV PIs may protect against disease in an unexpected way.
Recent in vitro research suggests that HIV-protease inhibitors may also demonstrate inhibitory activity against Pneumocystis carinii (responsible for PCP),1 against Candida albicans, a fungus that causes thrush,2 and against Toxoplasma gondii, which can infect the brain.3 Each of these pathogens can cause serious disease in immune-compromised persons.
It should be stated at the outset that the clinical implications of these test-tube observations are likely to be slight, mainly because the low-cost antibiotic trimethoprim/sulfamethoxazole (Bactrim) does a superior job of preventing PCP and toxoplasmosis and because those at risk for opportunistic infection (OI) ought to be receiving prophylactic treatment. But this research is interesting nonetheless because it sheds light on the complex interactions between bodies and drugs that lie just out of reach of medical certainty.
HIV protease is one of a family called aspartyl proteases, enzymes that cleave strings of amino acids at precise points with the scissors-like action of two matched aspartic acid molecules. Non-HIV aspartyl protease enzymes, such as renin, are known to be active in human as well as many other forms of life. The candidate compounds for HIV protease inhibitors were selected because they specifically blocked the target viral enzyme while virtually ignoring human ones. There have been suggestions that some of the unusual side effects associated with PI use, such as lipid abnormalities and insulin resistance, could perhaps originate with low-level non-specific inhibition of human enzymes, including human aspartyl proteases. The issue is unclear since other mechanisms and other drugs are also suspected. What the new research finds is that other organisms, including certain HIV-associated pathogens, may rely on PI-susceptible aspartyl proteases to perform essential functions.
As most people know, Pneumocystis carinii pneumonia (PCP) is a deadly lung infection that can strike immune-impaired people with fewer than 200 CD4 T-cells. A group of Italian researchers has reported inoculating Pneumocystis trophozoites onto human embryonic lung cells, then exposing them to several PIs and to Bactrim as a control. Each of the PIs, nelfinavir, indinavir, ritonavir, and saquinavir, demonstrated partial, dose-dependent pneumocystis inhibition with activity falling short of Bactrim at the concentrations tested. Although blood levels of PIs can far exceed the concentrations attained in this experiment, the amounts of drug that actually reach the alveoli of the lungs, where pneumocystis does its damage, are unknown. Though the PIs were not dramatically effective against PCP, evidence that pneumocystis is dependent upon an aspartyl protease suggests a potential new target for anti-PCP drug research.
A different group of Italian researchers explored the inhibitory effects of indinavir and ritonavir on the growth of Candida albicans. They established that a particularly virulent form of C. albicans associated with HIV infection produces a secretory aspartyl protease and that this protease is inhibited by the HIV PIs. Then, in an experimental mouse model of vaginal candidiasis, the researchers demonstrated that the PIs had a therapeutic efficacy comparable to that of fluconazole, a gold-standard antifungal. These findings, they suggest, help explain clinical observations of promptly improved oral candidiasis in patients who begin HAART -- even before their CD4 counts recover.
In the late '80s, after Bactrim prophylaxis was shown able to prevent most episodes of Pneumocystis carinii pneumonia (PCP), researchers turned their attention to preventing some of the second-line -- yet no less deadly -- opportunistic infections. One of these was toxoplasmic encephalitis, caused by Toxoplasma gondii, a protozoan parasite that can be acquired by contact with cat feces or uncooked meat. When an immune-compromised, parasite-infected person experiences CD4+ T-cell counts falling below 100, their risk of developing a brain infection with T. gondii increases. Fortunately, as was subsequently learned, Bactrim, when routinely taken to prevent PCP helps prevent toxoplasmosis, also.
A team of French researchers recently conducted a study to see if some of the HIV antiretroviral drugs have activity against the T. gondii parasite. They selected a particularly virulent strain of toxoplasma and grew it in a laboratory human cell line. Varying concentrations of the nucleoside reverse transcriptase inhibitors (NRTI) AZT, ddI, d4T, ddC, 3TC as well as the HIV protease inhibitors saquinavir, ritonavir, indinavir and nelfinavir were tested. All of the HIV drugs were compared to the standard treatment for toxoplasmosis, pyrimethamine plus sulfadiazine.
The NRTIs ddI, d4T, 3TC, and ddC had no effect on the growth of T. gondii. AZT had a mild inhibitory effect on the parasite at a relatively high concentration of 100 micrograms/mL. More significantly, there was no observed antagonism between AZT and pyrimethamine -- results that contradicted a 1989 report.4 Also, contrary to a 1997 report, ddI showed no anti-toxoplasma inhibitory activity.5 Overall, the NRTIs neither potentiated nor antagonized the standard toxoplasmosis treatment regimen.
Of the PIs, indinavir had an unremarkable inhibitory effect in line with that of AZT, and saquinavir killed toxoplasma at concentrations that also killed the human host cells. But, nelfinavir and ritonavir were standouts. Highly inhibitory to T. gondii at concentrations of about 10 micrograms/mL, each PI entirely blocked parasite growth at levels safely attainable in humans. Although no native T. gondii aspartyl protease has been discovered, this research suggests that, as with pneumocystis, there must be one at work and that it may be an attractive target for anti-toxoplasma drug development.
With the advent of HAART, the incidence of all OIs has dropped dramatically. Current OI treatment guidelines recommend that people with fewer than 200 CD4 cells take Bactrim as prophylaxis against PCP (also protective against toxoplasmosis) -- whether they are on HAART or not. Despite the success of combination therapy with PIs, not everyone with advanced disease is able to respond with T-cells rising to protective levels. Often it is because they have developed resistance to every available drug and cannot suppress the virus. For others, immune reconstitution remains stunted despite HAART-mediated control of viral replication. Until we know more about T-cell recovery, we must presume that people with very low T-cell counts remain at risk for developing any and all of the classic opportunistic infections that define AIDS.
These data might lead some to wonder if certain PIs should be prescribed for individuals with dangerously low T-cell counts. All things being equal, if there is added protective benefit from taking nelfinavir or ritonavir above and beyond their ability to suppress viral replication, should these drugs be preferred as standard of care for at-risk individuals?
It's not that clear. Keith Henry, an HIV clinician who is concerned about seeing rising rates of morbidity in his practice after several years of decline, doesn't think so. "It's interesting, but these test-tube studies are subject to a lot of spin. There's no substitute for good medical management, prophylaxis and finding a treatment regimen that suppresses virus."
Finding new and unexpected uses for existing drugs is not unusual. Thalidomide was introduced as a tranquilizer, became notorious as a teratogen, and is now being investigated in HIV care for treating apthous ulcers and wasting syndrome. Low-dose ritonavir has insinuated itself into an increasing number of HIV treatment regimens by virtue of its unsurpassed ability to inhibit a liver enzyme system and thus raise the blood concentrations of other PIs -- a trait unrelated to its antiretroviral properties.
Maybe it's not surprising that biologically active substances, when let loose in a complex system of chemical interdependencies, often have unexpected effects. Research is continuing into the mechanisms of treatment toxicity, including those that may be PI-related. Perhaps these reports of peripheral treatment benefits can help researchers unravel some of the complex interactions between HIV, the body, and the pharmacy.