Two experimental, immune-based therapies have entered preliminary trials in HIV-infected individuals in Australia. The therapies, PEHRD214 and LHRH, may boost the immune responses of people with HIV, according to laboratory and animal studies.
Human studies of PEHRG214 and LHRH exemplify the immunological focus of current HIV treatment research. Renewed interest in immune-based therapies has emerged as the limitations of available antiretroviral therapies have become apparent.
Antibody therapy
PEHRG214 is an antibody cocktail developed by a biotechnology company called Probe Australasia.
Laboratory studies have shown that PEHRG214 is able to destroy cell-free viruses as well as facilitate destruction of HIV-infected cells. Although the mechanism is not fully understood, research suggests that PEHRG214 targets functionally significant HIV protein sites which are not recognised by the human immune system.
PEHRG214 has been developed from goat antibodies and is the first animal-based HIV antibody preparation to be approved for human trials.
Associate Professor Andrew Lloyd, Chair of the Australian Immune-based Therapies Working Group of the National Centre in HIV Epidemiology and Clinical Research which is overseeing the study of PEHRG214, told aidsmap that the working group " had been very cautious and conservative in moving forward to a phase I study". He said Probe had provided a lot of in vitro data to justify the trial.
"We think it is worth a phase I study. There is enough data to suggest that PEHRG214 is likely to be safe and may be effective," Dr Lloyd said.
Recent research conducted in macaques has indicated that antibodies may play an important role in facilitating immune responses to virally infected cells and to free virus. Research presented at the 2001 Retroviruses Conference showed that particular variants of SIV with deletions in the V1 and V2 regions of the envelope protein gp120 (the M5 variant) could be controlled solely by antibody responses, leading to undetectable viral load or extremely low levels of viremia in macaques. Furthermore, subsequent challenge with more virulent variants of SIV did not result in viral load rebound, suggesting that the antibodies formed in response to an M5 variant also provide protection against other SIV strains.
Dr Lloyd said that PEHRG214 may prove to be a therapy which can stimulate this type of immune response.
The phase I safety and pharmacokinetic trial of PEHRG214 in HIV-infected people is being conducted by St Vincent’s Hospital, Sydney, and the Harvard Medical School, Boston, USA. The trial is open to individuals with advanced HIV disease and limited treatment options and, to date, three people at St Vincent’s have received an injection of PEHRG214. No adverse events have been detected at this stage. If preliminary dosing is shown to be safe, dose escalation will follow.
Regeneration of the thymus
The second experimental immune-based therapy for HIV being investigated in Australia is a synthetic form of LHRH (leutinising hormone releasing hormone). This synthetic hormone is a commonly used treatment for prostate cancer. Research has found LHRH blocks the sex hormones (estrogen and testosterone) and leads to regeneration of the thymus in animals and men with prostate cancer.
The thymus is the source of immune cells known as T-cells. The thymus is very active during childhood but its role declines dramatically during puberty. Researchers hope that LHRH will produce thymic regeneration which will, in turn, lead to production of new CD4 cells.
A pilot study of LHRH in HIV-infected individuals is being conducted by researchers at the Alfred Hospital, Melbourne. This trial will examine the impact of LHRH on the thymus and CD4 cell production. Participants must have been taking highly active antiretroviral therapy for at least six months, have viral loads below 10,000 and CD4 counts which have remained between 50 and 300. Duration of treatment is four months.
The major drawback with LHRH is that is causes impotence and loss of libido. Although these side-effects are fully reversible once treatment ceases, researchers have expressed concern that patients may not be interested in an experimental treatment which causes temporary loss of sexual functioning.
Caution
Researchers involved in the two studies were reluctant to ‘talk up’ the potential benefits of these experimental therapies, given the absence of any human safety or efficacy data. They urged caution regarding the possibilities of these therapies.