Genetically modifying CD4 cells to knock out the CCR5 coreceptor resulted in significant CD4 count gains and notable viral load reductions while off antiretroviral (ARV) therapy, according to new data from a clinical trial of Sangamo Biosciences’ zinc finger nuclease (ZFN) therapy SB-728-T, reported Thursday, March 8, at the 19th Conference on Retroviruses and Opportunistic Infections in Seattle.
After HIV binds to the CD4 protein on CD4 cells, the virus must then latch onto another receptor on the cell’s surface—either CCR5 or CXCR4. Usually, when people contract HIV, their virus starts off using the CCR5 receptor. Later on, as HIV disease progresses, the virus can switch to the CXCR4 receptor—this occurs in about 50 percent of treatment-experienced patients.
Selzentry (maraviroc), a U.S. Food and Drug Administration (FDA)–approved ARV, works by blocking the interaction between CCR5 and HIV, ultimately retarding the virus’s ability to infect CD4 cells. SB-728-T, a zinc finger DNA-binding protein transcription factor, goes one step further—it blocks the gene responsible for making CCR5, mimicking a naturally occurring human mutation that renders individuals largely resistant to the virus.
This mutation, dubbed CCR5 delta-32, appears to have no harmful effect in the human body. In addition, a study published in the journal Blood in December 2010 reported that an HIV-positive person with leukemia was cured of HIV when he received a bone marrow transplant from a “matched” donor who had inherited this delta-32 CCR5 mutation from both parents. (When the mutation is inherited from one parent, CCR5 is produced, but at low quantities and is associated with slower HIV disease progression. When the mutation is inherited from both parents, which is very rare, little or no CCR5 is expressed on CD4 cells, rendering the cells impervious to forms of HIV that use the CCR5 receptor to enter cells.)
Sangamo’s gene therapy approach has both therapeutic and curative potential. At present, only Sangamo’s therapeutic-focused CCR5-knockout SB-728-T has entered clinical trials.
Therapy involves removing CD4 cells from patients’ blood, treating the cells with SB-728-T to knock out the CCR5 gene, multiplying the cells in the lab, then transplanting the HIV-resistant genetically modified cells back into the body.
Fully curing HIV will be a bit more complicated, as it ultimately requires replacing the entire CD4 population with HIV-resistant cells, not merely creating a small reservoir of protected cells. A curative approach will likely involve removing and treating stem cells with CCR5 and possibly CXCR4 knockout genes, administering high-dose chemotherapy to wipe out the existing HIV-susceptible immune system, followed by transplanting the modified stem cells to rebuild an immune system that is resistant to the virus.
In one therapeutic study reported at last year’s CROI in Boston, Jay Lalezari, MD, of Quest Clinical Research found that infusions of CD4 cells modified using SB-728-T were safe, disseminated throughout the body and associated with CD4 cell increases in a small group of HIV-positive patients with “immune-discordant” responses to ARV therapy—in other words, they had undetectable viral loads but limited CD4 gains despite several years of treatment.
Another study, detailed in September at the 51st Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) in Chicago, reported on six patients—none of whom was an immune-discordant responder to ARV treatment—who underwent a treatment interruption one month after receiving SB-728-T-modified infusions of their own CD4 cells. In three of the six subjects, significant viral load reductions were documented during the treatment interruption phase of the study. Viral load levels also went undetectable in one patient.
Pablo Tebas, MD, of the University of Pennsylvania and his colleagues provided an update on the six study volunteers who underwent a treatment interruption following SB-728-T treatment, along with the 15 immune-discordant ARV treatment responders. All patients received a single infusion of 5 billion to 30 billion CD4 cells modified with SB-728-T.
The average age of the immune responders—those who were otherwise responding well to ARV treatment—was 46. All were male with CD4 counts averaging more than 900 at the start of the trial. Among discordant responders, the average age was 48 and they averaged 335 CD4 cells at the start of the study; two of the 15 were women. The average CD4:CD8 cell ratio—a measure of immune system balance that is typically low in people living with HIV—was 1.4 among the six immune responders and 0.7 among the 15 discordant responders at baseline (normal is generally 1.0 or higher).
SB-728-T treatment continues to be safe and well tolerated. Severe adverse events were rare; the vast majority were mild-to-moderate in intensity and were mostly seen within 24 hours following infusions. The most common infusion-related side effects were chills, fever, headache and excessive sweating.
CD4 cell increases were noted in both groups. In the immune-responder group, CD4s shot up more than 1,500 points following the infusion. By day 30, CD4 cell counts in this group were still 1,000 points above their pre-infusion levels.
Less substantial, but statistically significant, gains in CD4 cells were seen in the discordant responder group as well. After initially seeing their CD4 counts increase 500 points following infusion, the group saw their average numbers of CD4s consistently remain above pre-infusion levels for about a year.
CD4:CD8 ratios normalized in the discordant responder patients and increased in the responder patients.
Tebas and his colleagues also noted elevated levels of cellular messengers—the CD4 cell cytokines IL-2, IL-7 and IL-15—immediately following the infusions, which may likely explain post-infusion symptoms and, more positively, the rapid expansion of CD4 cells.
The researchers also reported that the ZFN-modified CD4 cells could be detected in blood samples for over a year, ranging from 90 days in one patient to more than 700 days in another. Also of importance, the modified cells showed evidence of traveling and taking up residence in gut tissue, an important HIV reservoir in the body.
Tebas reiterated the results of the treatment interruption experiment—conducted only in the immune responder patients—reported at the September ICAAC conference. After therapy was stopped, viral loads initially increased in all patients. This was followed by viral load drop—reductions ranged from 0.8 to more than 2.0 log—in three of the six patients.
The one patient who saw his viral load decrease to undetectable levels entered the study carrying the natural CCR5 delta-32 mutation on one copy of his CCR5 gene, with the infusion facilitating an even more substantial HIV-CCR5 blockade. This finding, coupled with observations from the other five patients who initiated a treatment interruption, suggested to Tebas and his colleagues that control of viral load in the absence of therapy correlated with levels of circulating CD4 cells in which both copies of the CCR5 gene (bi-allelic) underwent modification as a result of SB-728-T therapy.
Finally, Tebas said that levels of circulating proviral DNA in CD4 cells—a measure of the viral reservoir—increased four times and nine times in two of the six patients, respectively, during the treatment interruption. However, these increases reversed when ARV therapy resumed.
In conclusion, Tebas said that data from these studies involving discordant responders and immune responders is “promising and warrants further evaluation. Further clinical development of SB-728-T,” he added, “will aim to maximize patient exposure to CCR5 bi-allelic modified CD4 cells in two ongoing clinical trials.”
One of those trials, he specified, will focus on patients carrying the natural CCR5 delta-32 mutation on one copy of their CCR5 genes. The second study will employ chemotherapy to pre-condition the immune system and potentially increase expansion of the CCR5-modified CD4 cells.