CD4 is a receptor on the surface of T-helper cells which HIV binds onto in order to gain entry into the cell. An artificial (or recombinant) version of CD4 (called rCD4) has been manufactured. To make rCD4 stay in the body longer an alternative molecule consisting of rCD4 and antibody has been prepared: rCD4-IgG.

The theory behind this treatment is that significant quantities of HIV in the blood will bind onto these artificial CD4 molecules rather than onto the real CD4 molecules on the surface of human cells. Essentially, the recombinant molecule should act as a decoy, mopping up viruses in the bloodstream.

Current use

Recombinant CD4 and associated treatments such as rCD4-IgG are unlicensed, experimental drugs. They are not available in Britain.

Recombinant CD4 has been shown to work in the test-tube against laboratory strains of HIV, but researchers found that very few `wild' strains of HIV taken from infected people bound onto the recombinant molecule.

Some researchers believe that the theory behind this treatment is fundamentally flawed because the body automatically produces antibodies against HIV's envelope protein gp120 which bind onto the virus just like recombinant CD4 is meant to, but these antibodies do not seem to affect the progress of the disease. Similarly, researchers have found that people with HIV naturally have soluble CD4 circulating in their bloodstream at levels which remain high even in relatively advanced disease, suggesting that it has little or no effect on disease progression.

Phase I trials in people, in which rCD4 was given by drip or injection, have found no evidence of immunological or antiviral effects. No significant side-effects from these CD4-based treatments have been reported.

Researchers developed a form of recombinant CD4 which is linked to a poison, Pneumonas aeruginosa exotoxin A (PE40). This is also known as Alvircept Sudotox. The theory behind this treatment was that this molecule, known as rCD4-PE40, would bind onto and kill cells infected with HIV which have the virus's gp120 protein on their surface. This worked in laboratory tests and did not kill uninfected cells. However, human trials found little evidence that the treatment had any effect on levels of HIV in the blood. The molecule is probably too big to be able to cross the blood-brain barrier and act against HIV-infected cells in the central nervous system. It is no longer being developed by its manufacturer, Pharmacia & Upjohn.

However, Sequus Pharmaceuticals and the University of California have developed a liposome which bears the CD4 molecule. A liposome is a fatty substance which protects the rsCD4 from being broken down in the body. In vitro studies have found evidence of anti-HIV activity and research is continuing.

Use in combinations

Test-tube studies suggest that the combination of AZT and rCD4 is synergistic against HIV in a wide range of cell lines, without harming uninfected cells.

Current developments

Another strategy under investigation is the use of anti-CD4 monoclonal antibodies. This is an artificial antibody which resembles an antibody to the CD4 receptor. The theory is that when it is injected into the body, the immune system will produce new antibodies against the monoclonal antibody. These new antibodies should closely resemble the CD4 receptor itself, and so may also bind onto HIV's gp120 protein and prevent the virus from binding to cells that carry the `real' CD4 molecule. This approach to treatment is also known as anti-CD4 idiotype vaccination. A preliminary trial has suggested that it only rarely causes side-effects, and may delay clinical or immunological disease progression. The molecule is called IO4Ta, made by Immunotech.

Key research

Recombinant soluble CD4 (rCD4) molecules are soluble, secreted forms of CD4 receptor molecules which lack its transmembrane and cytoplasmic portions. rCD4-IgG are synthetic, antibody-like hybrids of rCD4 molecules attached to an immunoglobulin (IgG). CD4-IgG have retained key properties of both rCD4 and an immunoglobulin (IgG). They bind to the gp120 of laboratory strains of HIV and block infection of T cells and monocytes in vitro. The attached immunoglobulin (IgG) gives the molecule a longer serum half-life than soluble CD4 alone. CD4-PE40 is a recombinant protein produced by genetic engineering technology that contains the HIV binding region of human CD4 linked to the active domains of Pseudomonas aeruginosa exotoxin A.

In separate in vitro studies Deen, Fisher, Hussey, Smith and Traunecker reported that when tested against laboratory strains of HIV-1, rCD4 binds to the HIV envelope protein gp120 and thereby blocks viral infectivity, replication and HIV-virus induced cell fusion (syncytium or giant cell formation).

Moore and Daar both reported that rCD4 and CD4-IgG were largely ineffective in vitro when tested against primary isolates of HIV (i.e. those freshly obtained from an HIV-infected individual) rather than laboratory strains such as HIV-IIIB. The investigators reported that this ineffectiveness resulted from low binding affinity between rCD4 and the gp120 from primary HIV isolates. The investigators further suggest that this low binding affinity may account for the lack of efficacy seen to date in clinical trials of rCD4 and CD4-IgG. However, in a separate study Ashkenazi found that the CD4-binding affinities of rgp120 from strains with markedly different CD4 sensitivities were essentially the same and only small differences were observed in the kinetics of CD4 binding. These results suggest that the lower sensitivity of primary HIV-1 isolates to neutralization by CD4-based molecules is not due to lower binding affinity between soluble CD4 and free gp120.

Callahan reported that the binding of soluble CD4 molecules to viral gp120 is reduced in the presence of anti-gp120 antibodies which compete with soluble CD4. Since anti-gp120 antibodies occur in HIV infection in vivo and since the presence of these blocking antibodies does not seem to affect the development of disease, soluble CD4 seems unlikely to suppress HIV infection by simply blocking envelope binding. However it is possible that soluble CD4 has additional effects on virus neutralisation, even in the presence of blocking antibodies. It may be possible to overcome the blocking effects of anti-gp120 antibodies by using much higher concentrations of soluble CD4.

Ward reported that pre-treatment with CD4-IgG protected chimpanzees from infection with HIV-1 IIIB. Chimpanzees received CD4-IgG 5 mg/kg intravenously 8 hours and 1 hour before inoculation with cell-free virus and then received CD4-IgG 2.5 mg/kg/day by intramuscular injection for 6 weeks. The investigators suggested that CD4-IgG may therefore effectively prevent perinatal HIV transmission.

Meng conducted a Phase I trial in which AZT (300-600 mg/day) and CD4-IgG (300-3000 µg/kg twice weekly) were give in combination to 41 HIV-positive people in 5 dose regimens. The antiviral activity of the combination was no better than reported previously with AZT alone.

Schooley and Kahn enrolled a total of 60 people with AIDS and ARC in two phase I/II escalating-dose safety and pharmacokinetic trials of rCD4 (manufactured by Biogen and Genentech respectively). Participants received rCD4 doses ranging up to 300 mg/kg/day intravenously, intramuscularly, or subcutaneously for up to 11 weeks. No significant toxicities or side-effects were observed. No significant changes in CD4 or CD8 counts or p24 antigen levels were observed in either study.

Johnson evaluated the interactions of soluble CD4 and AZT against HIV-1 replication in vitro in a variety of cell types. The combination inhibited HIV-1 synergistically over a broad range of drug concentrations and in all cell types tested without additive cytotoxicity.

Husson conducted a phase 1 study to determine the safety and pharmacokinetics of recombinant soluble CD4 (100, 300 and 1000 µg/kg/day by continuous IV infusion) in children with symptomatic HIV infection. After an initial 12 weeks of treatment with sCD4 alone, ddI (90 mg/m² every 8 hours) was added. Combination therapy was continued in patients in whom it was well tolerated. During the 12 weeks of treatment with sCD4 alone and during subsequent sCD4 plus ddI combination therapy no significant toxic reaction attributable to sCD4 or ddI was observed. Low-level anti-CD4 antibodies developed in 2 participants. Steady-state sCD4 levels increased proportionately at higher doses. The CD4 cell counts and serum p24 antigen levels did not provide evidence of antiviral activity.

Collier, Hodges and Davey (1991) enrolled a total of 57 people with AIDS and ARC with CD4 counts below 500 in three separate phase I escalating-dose safety and pharmacokinetic studies of rCD4-IgG. Participants received doses ranging up to 3,000 mg/kg/day intravenously and 6,000 mg/kg/day subcutaneously with and without AZT 500 mg/day. No significant toxicities were observed. No significant changes in CD4 count or p24 antigenaemia were reported.

Mulligan conducted a phase I trial of CD4-IgG in 6 HIV-positive individuals with documented in vitro sensitivity to CD4-IgG and CD4 counts between 50-200. There was no evidence of drug toxicity and no participants developed antibodies to CD4-IgG. CD4 counts, neopterin levels and beta₂-microglobulin levels remained unchanged. Mean titre of infectious plasma virus was reduced by at least 5-fold in 2/6. 3/6 had a >25% reduction in p24 antigen levels.

Shearer treated 6 HIV-infected pregnant women in the third trimester of pregnancy with rCD4-IgG (1 mg/kg by intravenous bolus) either at onset of labour, or 1 week prior to delivery and at onset of labour. Pharmacokinetic data indicate that the mean peak serum concentration of rCD4-IgG in pregnant women is 16.0 µg/mL and that rCD4-IgG crosses the placenta. There was no evidence of toxicity or immunologic or clinically significant laboratory abnormalities having been seen in mothers or infants. None of the infants has shown evidence of infection with HIV.

In separate in vitro studies Ashorn, Chaudhary, Pitts and Rubino showed that CD4-PE40 selectively kills cells expressing gp120. It is inactive against cells latently infected with HIV and uninfected cells. The investigators note that the size of the CD4-PE40 molecule (60kD) may be too large to cross the blood-brain barrier.

Till reported the coupling of CD4 with other toxins such as ricin.

Davey (1992) conducted a phase I dose-ranging study of single intravenous bolus doses of intravenous CD4-PE40 (1, 5, 10, 15 or 45 µg/kg) in 24 HIV-infected individuals with CD4 counts below 500 (mean CD4 count was 215). The half-life averaged 2 hours. Anti-PE40 antibodies developed in 12/24 within 3 weeks. Davey (1993) re-challenged 17 with monthly doses for 2 months, then weekly doses for 6 weeks. No relationship was found between prior formation of antibodies to PE40 and toxicity. The major toxicity consisted of transient elevations in hepatic transaminases characteristically peaking 48 hours after intravenous bolus infusion. No consistent beneficial or harmful effects on CD4 count, CD4 percent, serum p24 antigen, beta₂-microglobulinaemia or plasma viraemia were observed at the doses tested.

Fiscus enrolled 55 individuals with CD4 counts below 300 in a dose-ranging study (ACTG 201) of weekly CD4-PE40. The maximum tolerated dose was 320 µg/m² when given once per week, and 64 µg/m² when given five times per week. LFT elevations were the dose-limiting toxicity. In the group treated once weekly for 8 weeks, no significant differences in CD4 counts, p24 antigen levels, plasma cultures, quantitative cell cultures or quantitative RNA assays were observed. No patients developed anti-CD4 antibodies. 50% developed neutralizing antibodies.

Wathen reported that in tests of sera from individuals treated with CD4-PE40, no antibodies to CD4 were detected, but 18/36 developed antibodies against the PE40 domain. Although most individuals developed neutralising antibodies, only 8/36 experienced 10-fold increases in neutralising titre over the course of a 10 week study.

Sutor (1994) vaccinated 10 people who had CD4 counts between 200 and 400 with alum-precipitated anti-CD4 monoclonal antibodies (0.6, 1.2 or 2.4 mg doses) in a phase I trial. The vaccine was well-tolerated. 8/10 recipients experienced an increase in CD4 count and an additional increase was observed in 6/6 who received a booster vaccination. Serum antibodies from 4 participants showed an augmented capability to inhibit gp120/CD4 interactions.

Sutor (1996) enrolled 158 people with CD4 counts between 350 and 500 in a trial comparing alum-precipitated anti-CD4 monoclonal antibodies (1.2 mg), with a 0.6 mg booster after 3 months, versus placebo. The vaccine was well-tolerated in all but three recipients, who developed rash or oedema. Treated participants demonstrated increased neutralising antibody titres, and perfomed significantly better in triparamtetric analysis of clinical and immunological study endpoints and p24 antigen assays.

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