By encapsulating HIV drug molecules into tiny polymer particles that slow-release drug when they are injected, researchers are working on the next step in simplifying HIV therapy: injectable HAART you could take once a month.
The company, and the drug, that has travelled furthest along this route are Tibotec/Johnson and Johnson and their as-yet-unlicensed NNRTI drug rilpivirine (TMC278). Rilpivirine was chosen because in its oral-tablet form it has a long half-life and a high bioavailability, which means a single daily dose of only 25mg (compared, for instance, with 600mg in the case of Tibotec’s protease inhibitor darunavir).
Dr Gerben van t’Klooster presented the findings on Wednesday at the Fifteenth Conference on Retroviruses and Opportunistic Infections in Boston.
Tibotec formulated TMC278 as a suspension of tiny slow-release particles. They were not forthcoming about exactly how these were made, except to say that it involved something called NanoCrystal technology. These particles had an average diameter of 200 nanometres (one five-thousandth of a millimetre), which is comparable to the size of the HIV virus (120nm).
This suspension was then, in a number of different experiments, dosed as a subcutaneous or intramuscular injection in rats (at a dose of 20mg per kg) and dogs (at doses of up to 300mg a day).
A single injection of one particular formulation was then given by subcutaneous and intramuscular injection to HIV-negative human volunteers at doses of 200, 400 and 600mg of drug.
Rilpivirine was slowly released, giving sustained, measurable levels of drug for two months in rats and as long as six months in humans. In animal studies subcutaneous injection provided longer-lasting drug levels than intramuscular injections, but in humans there was no difference. This is a good thing as human volunteers experienced a fairly high level of injection site reactions – hard lumps (induration), pain and swelling – in subcutaneous injections which was absent in intramuscular ones.
Presenter Gerben van t’Klooster said the peak drug concentration was achieved within three hours of injection. Levels in the body after a single dose fell to the IC90 minimum effective concentration of 94ng/ml (nanograms per millilitre) of rilpivirine within a few days. However the experiments in dogs had shown that with repeated dosing a ‘steady state’ level of drug was achieved in the body. Van t’Klooster showed an as-yet theoretical PK model which showed that once this steady state was achieved, a monthly injection should be sufficient to ensure that rilpivirine concentrations did not fall below the IC90 level.
Van t’Klooster said the next stage was to concentrate rilpivirine into the nanoparticles more efficiently so that injection volume could be reduced.
He added: “I hope I’ve convinced you of the opportunity that exists for truly infrequent dosing of antiretrovirals – in both a prophylactic and a therapeutic setting,” dropping a hint that Tibotec is also interested in this slow-release formulation for use in pre-exposure prophylaxis or microbicides.
He said that Tibotec were actively looking for molecules to pair with rilpivirine so that a truly injectable combination therapy could be found. He said that drugs such as darunavir needed daily doses too large for a slow-release formulation to be feasible, because of intolerably large injection volumes.
However another group based at Creighton University in Omaha, Nebraska, has succeded in creating slow-release nanoparticles containing the drugs lopinavir, ritonavir and efavirenz. So far they have only tested these particles’ drug-releasing properties by suspending them in a medium in a laboratory dish. Maximum drug levels in the medium were reached by six days, but at 30 days drug concentration in the medium was still, even with regular changes of medium, over 30mg/ml of drug. They also did experiments to show that the nanoparticles were readily taken up by human monocyte‐derived macrophages, a kind of immune-system cell.
A couple of other posters detailed ways of using nanoparticles. In another experiment from Creighton University, scientists succeeded in loading indinavir into nanoparticles then getting bone-marrow-derived macrophages (BMMs), another kind of immune cell, to absorb them. These were then injected into mice that had had HIV-related encephalitis induced. The BMMs migrated preferentially to sites in the brain where nerve cells were being destroyed due to HIV-related inflammation. Conversely, they were not found in brain areas without inflammation. This model provides an exquisitely precise way of targeting drugs that normally cross the blood-brain barrier inefficiently to the sites in the brain that most need them.
Finally, a team from the University of North Carolina attached a normally inactive CCR5 inhibitor to gold nanoparticles, and thereby restored its anti-HIV activity. The point of doing this was to create large gold-drug complex molecules that would act like, and interact with, large viral proteins, and eventually to develop a mechanism to introduce agents into cellular spaces that have proved difficult to target with small-molecule drugs. An example includes vif, the viral infectivity factor, an HIV accessory protein that has been a tantalising target for HIV drug delivery for years but which has so far eluded inhibition.