The Hydroxyurea Steamroller
Perhaps no other experimental HIV therapy has been talked about more these past months than hydroxyurea (HU). Much of the attention concerns three reported cases of HIV-infected individuals who had been on an HU-containing combination regimen for a period of time, then stopped all treatment and have shown no signs of viral replication after many months off therapy. There is, in addition, a wealth of information from ongoing clinical studies. The new trial data may have even more impact in the potential use of this drug as a new treatment for HIV infection.
When it comes to new anti-HIV therapies currently in the development pipeline, HU stands out as a unique case. An old anticancer drug, now off-patent and relatively inexpensive, its target is not HIV itself, but the immune system's cells attacked by the virus. In theory, it acts by blocking a cellular enzyme (ribonucleotide reductase) that helps produce the DNA building blocks, or "nucleotides," used by both the cell and by HIV as it creates its DNA genetic template within newly infected cells. By depleting the pool of available nucleotides, hydroxyurea facilitates the incorporation into the viral DNA structure of defective nucleotides derived from drugs such as ddI. A noteworthy characteristic of HU is that it has a significant protective effect on "resting" (i.e., nonactivated) CD4 T-helper cells, in contrast to the antiviral drugs. Curtailing HIV's chance to infect resting cells would reduce the long-lived latent viral reservoir (see "HIV Resurrection," in this issue) and shorten the timeline for "eradicating" HIV from the system.
Off Drugs and No Viral Rebound
The most impressive of the three case reports was presented by Dr. Franco Lori, codirector of the Research Institute for Genetic and Human Therapy (RIGHT), at the Sixth European Conference on Clinical Aspects and Treatment of HIV Infection in Hamburg, Germany. A recently infected German patient with a viral load of 85,000 copies/ml was treated with the triple combination ddI, HU and indinavir. The patient's HIV quickly became "undetectable" -- i.e., his viral load was too low for the test to quantify. Because he developed a severe case of hepatitis A, he had to be taken off the HIV drugs 144 days after he started treatment. After being off drugs for three weeks and remaining BLQ (below the limit of quantification), he went back on treatment for two months. Last Christmas, the patient decided to refuse further treatment. Amazingly, his plasma viral loads have now remained below the limit of quantification for over ten months off treatment, although he does exhibit very low levels of HIV in his lymph nodes (see below). According to the patient's physician, Dr. Heiko Jessen of Berlin, "We continue to closely monitor this man's progress. We take blood twice a month, and we will do another lymph node analysis in March."
The other two cases were reported by Dr. Jorge Vila, who has been conducting HU studies based in France.(1) The main difference between these two patients and the German patient is that they had very low initial viral loads: 676 and 1120 copies/ml. The two patients were treated with ddI and HU for 12 months. They then agreed to stop all treatments. At the time they stopped all treatments, HIV RNA levels were too low to quantify in both blood and lymph tissue. However, very low levels of proviral DNA remain in the lymph nodes. (Residual proviral DNA represents latent HIV genes within cell nuclei. Most of it is considered defective, but some may be replication-competent.)
Over two years later, HIV RNA levels in the plasma and lymph tissue remain BLQ. Both patients still have trace amounts of proviral DNA in lymph nodes. Dr. Vila asserts that this provirus is defective and cannot replicate properly and that no infectious virus could be grown from it in his lab.
The experience of these three patients could represent a significant breakthrough. These reports' main limitation, besides their obvious nonrigorous, anecdotal nature, is that all three patients were treated in very early infection, and two of them had very low levels of HIV RNA in the first place. The RIGHT group, which includes Dr. Jesson, and codirector Dr. Julianna Lisziewicz as well as Dr. Lori, believes that the German patient still harbors virus, but that it doesn't come back. Dr. Vila aruges in his letter to The Lancet that the protective effect of HU in resting cells is a plausible explanation for the lack of viral rebound seen in his two off-treatment patients.
Hydroxyurea and ddI Resistance
Because HU acts on the cell and not the virus, resistance is not a significant problem. Cells do not mutate as quickly as HIV does. Furthermore, data from clinical studies with the combination of ddI and HU show that, even after HIV mutates and becomes ddI-resistant, suppression of the virus continues. In the longest controlled study yet of ddI versus ddI/HU,(2) 57 patients with CD4 counts between 250 and 500 were recruited in 1995. Nineteen of the volunteers were randomized to ddI monotherapy and 38 to ddI/HU. At week 24, HIV viral load had dropped .78 log (83%) in the ddI monotherapy arm and 1.32 log (95.2%) in the combination arm. The ddI monotherapy arm was then dropped, and 34 participants in the combination arm were evaluated at week 40. Mean plasma viral load drop from baseline remained essentially the same during these 16 weeks.
An analysis of the HIV genotype at week 24 showed that two out of eight participants in the ddI monotherapy group had mutations associated with ddI resistance (positions 65 or 74 of the reverse transcriptase gene). But ddI resistance mutations (position 74 and in one case, 184) also appeared in 6 of the 11 participants on ddI/HU who were analyzed.
Additional test-tube experiments with laboratory-created ddI-resistant virus showed marked inhibition of viral replication in the presence of ddI/HU, at concentrations similar to the ones used in the clinical studies. In other words, ddI remained effective as long as HU was present.
A Canadian study had already shown that ddI-experienced volunteers who added HU to their regimen responded in ways comparable to persons who start ddI and HU together.(3) More recently, an 80-person (ddI-naïve) study in the U.S., sponsored by the American Foundation for AIDS Research, compared a ddI monotherapy arm to a ddI/HU arm for 12 weeks (Table 1). Those in the ddI monotherapy arm that were able to tolerate ddI and had an antiviral response then added HU for 12 additional weeks.(4) The results indicated that those patients in the arm that added HU after week 12 experienced additional suppression, almost, but not quite, "catching up" with the group that started on the combination.
New 24-week data on the Swiss study of d4T/ddI/HU versus ddI/d4T/placebo (see Treatment Issues, Feb. 1997) show that 84% of the patients continuing HU were BLQ (using a quantification limit of 200 copies/ml) at week 24. Of those in the ddI/d4T/placebo arm who were considered "poor responders" at week 12 and thus had HU added to their regimen, 55% became "undetectable."(5)
At least one small, controlled study in Aviano, Italy, found no added benefit in the ddI/HU combination over ddI alone in AZT-experienced patients after 24 weeks of treatment.(6) The study had 21 patients, of which only 14 (8 in ddI monotherapy, 6 in ddI/HU) were evaluated. One patient in the ddI group developed severe allergic dermatitis, whereas one patient in the ddI/HU group had a milder cutaneous (skin) rash, both of which required interruption of treatment. One patient in the ddI arm and four in the ddI/HU arm were lost to follow-up. There were two occurrences of grade 2 leukopenia (low total white blood cell counts) and two of alopecia (hair loss) in the ddI/HU arm. The high incidence of side effects may be due in part to the dose of HU used in this study: 1,500 mg divided into three daily doses. This daily total is greater than the dosages used in most other studies, which range from 1,000 to 1,200 mg.
Combination With a Protease Inhibitor
One aspect that has been common to all controlled HU studies is the lack of a significant increase in CD4 counts. In fact, in some instances a slight decrease occurred. Researchers believe that this is a result of an HU-induced reduction in DNA synthesis. This inhibition could translate into lower levels of cell multiplication.
The RIGHT investigators have interpreted HU's "cytostatic" effect as something positive, since active cell proliferation results in more HIV being created. The objective, from this perspective, is to reduce HIV's target cells for the longest time possible, so as to "starve" the virus. The dilemma here is that you want to restore the immune system. As it happens, those with the most depleted immunity also exhibit the lowest CD4 improvement with the ddI/HU regimen.
Results from a new pilot study from Germany presented at the European Conference in Hamburg showed a significant CD4 increase as well as measurable immune restoration when a protease inhibitor (indinavir, or nelfinavir in cases of intolerance) was added to the ddI/HU combination.(7) Baseline information and trial results are summarized in Table 2 below. This pilot study indicates that a protease inhibitor increases the potency of the combination significantly, further suppressing the virus and thus allowing CD4 counts to bounce back. Results were similar to those obtained by standard therapy with three antiviral drugs. Dr. Jessen was the chief investigator in this pilot study. He commented, "I don't have large numbers, but I have a very good feeling about this drug. It seems to work in everybody who is adherent, even in those who have HIV that is ddI-resistant."
An important feature of this study is that, in order to determine HIV RNA and DNA levels in the lymph nodes, researchers took out a complete lymph node from the patients, instead of using the more limited biopsy usually performed. In the sole patient out of eight evaluated for HIV RNA in the lymph nodes who was detectable, the PCR assay gave only the slightest positive signal for HIV RNA, and no proviral DNA was found integrated into his cells' genes. This was the patient referred to above. His lymph node was extracted three weeks after therapy ended.
It is, however, the immunological substudy that yielded the most surprising results. For this substudy, researchers selected eight patients with viral loads below the limit of quantification and normal CD4 counts (median was 700) after being treated with the triple combination. A separate cohort of eight early infection patients with similar characteristics who chose not to be treated was used as a control group. Investigators from the Center for Blood Research in Boston analyzed standard immunological functions, such as responses to flu antigen and HIV envelope protein. They also analyzed a new marker known as CD3-zeta expression,(8) which is crucial for signaling and activating the CD8 cells responsible for killing HIV-infected cells. The researchers found that, while these immune functions were normal in the treated group, they were abnormal in the untreated control group. This difference was statistically significant, leading the researchers to conclude that even early infection patients have immunological impairment and that this impairment can be reversed with the ddI/HU/protease inhibitor combination.
Dr. Jessen's study is really just a compilation of the experience of his first 17 patients. All might not turn out so rosy. For example, one of his patients with acute HIV infection permanently went off therapy after three months and had a viral rebound, unlike the individual mentioned at the start of this article, who stayed on therapy for a longer period. More rigorous trials are coming: RIGHT is currently planning a 150-patient study next year of a triple combination including HU plus ddI and a protease inhibitor. The protocol for this U.S.-based study is still under development.
One potential problem that researchers studying HU feared was bone marrow suppression. This had been a problem with cancer patients. Data from several past and ongoing studies as well as anecdotal experience from individuals who have been using HU document that side effects have occurred, in some cases requiring interruption of treatment. A limited number of patients have experienced some degree of alopecia and leukopenia. One patient in a Spanish study developed pancreatitis, and in at least one uncontrolled study in the U.S., patients had to stop treatment due to major drops in CD4 counts and total lymphocytes (see Treatment Issues, Sept. 1995). Evidence suggests that doses of 500 mg three times a day are problematic, especially in patients with low CD4 counts
Dr. Jessen in Berlin uses weight-adjusted doses of 250 to 400 mg three times a day and claims to have observed no significant side effects. "We need a dose-ranging trial," he notes. "Maybe you only need a low dose." His study of HU with ddI and a protease inhibitor found that most of the toxicity came from indinavir (see Table 2).
In the controlled studies, treatment with 1,000 mg or 1,200 mg daily has been well tolerated. It is noteworthy that in some of the studies with ddI, like the AmFAR study mentioned above, it was intolerance to ddI that forced some patients to drop out after the first weeks. It is important that individuals taking hydroxyurea have their blood counts (white blood cells and platelets) monitored frequently to detect any toxicity.
Bristol-Myers Squibb, maker of both ddI and HU, continues its financial support of independent investigators' studies of these drugs in several combinations, some including one or more protease inhibitors. Several protocols for different populations (early and primary infection, treatment-naïve, and salvage) are currently under way in the U.S. and other countries. ACTG-307, which compares ddI/HU for six months versus ddI alone for three months with HU added for the second three months, is close to reaching its recruitment goal of 132 patients nationwide. Trial site information can be obtained by calling 800/TRIALS-A.
Adding hydroxyurea to ddI-containing regimens offers the promise of stabilizing disease progression and viral loads even when they are somewhat above the limits of quantification. This is because the emergence of drug resistance is not that much of an issue with such regimens. ddI plus hydroxyurea is also comparatively inexpensive and could be used in areas where cost is a major obstacle to treatment. Further, larger studies will demonstrate how much of this potential can be fulfilled.
1. Vila J et al. The Lancet. Aug. 30, 1997; 350(9078):635-6.
2. Lori F et al. AIDS Research and Human Retroviruses. Nov. 1, 1997; 13(16):1403-9.
3. Montaner JS et al. Journal of Infectious Diseases. April 1997; 175(4):801-6.
4. Hellinger JA et al. 37th ICAAC, Toronto, Canada. Sept. 28-Oct. 1, 1997. Poster I-127.
5. Rutschmann OT et al. Abstract, 6th European Conference on Clinical Aspects and Treatment of HIV Infection. 1997.
6. Simonelli C et al. AIDS. Aug. 1997; 11(10):1299-1300.
7. Jessen H et al. Abstract, 6th European Conference on Clinical Aspects and Treatment of HIV Infection, 1997.
8. Trimble LA and Lieberman J. Blood. Jan. 1998; 91(15):(forthcoming).
This article was provided by Gay Men's Health Crisis. It is a part of the publication GMHC Treatment Issues. Visit GMHC's website to find out more about their activities, publications and services.