PrEP at CROI 2018 (part 2): Animal studies for future drugs
Simon Collins. HIV i-Base
Part 2 of our PrEP reports from CROI 2018 summarise early stages for promising approaches to future PrEP.
Tiny dose EFdA (MK-8591) protects against rectal SHIV
One of the most exciting potential HIV drugs with an indication for both treatment and prevention is an NRTI called MK-8591. This compound has high potency, retains activity against many NRTI-resistant mutations, and has the potential for formulation in a slow release removal implant that would provide drug coverage for a year.
New data at CROI 2018 included results from an animal PrEP study showing that progressively lower doses of EFdA still remained highly effective as PrEP. 
This added to results first presented as a late breaker at the IAS conference last year in Paris showing that weekly dosing (3.9 mg/kg, equivalent to 10 mg/week human dose) of EFdA protected all eight animals following weekly rectal exposure, compared to rapid infection in controls. 
The new results at CROI 2018, presented by Martin Markovitz from the Aaron Diamond AIDS Research Centre in New York, continued this study in the same animals, but using steadily lower doses: 1.30, 0.43, 0.10 and 0.025 mg/kg. Each animal received up to four challenges (a week after dosing), with a 4-8 week washout period before moving to the next lower dose.
At the 1.3 and 0.43 mg/kg doses, all eight animals continued to be protected after multiple SHIV rectal challenges. At 0.1 mg/kg, 2/8 animals did become infected (after the third and fourth challenge), although protection was still highly significant compared to control animals (p=0.004). Post infection viral load in these two animals was significantly lower (by 3 to 4 log copies/nL) compared to control animals. At the lowest 0.025 mg/kg dose, all four remaining animals did become infected.
Mean levels of intracellular MK-8591-TP at the time of challenge were 282 and 102 fmol/million PBMCs at the 1.3 and 0.43 mg/kg dosing levels, respectively (compared to 810 at the original 3.9 mg/kg dose). At the 0.1 mg/kg dose levels were not able to be detected, either in plasma or in cells, but were estimated at 24 fmol/mil PBMCs based on linear dynamics. All animals had wild-type HIV after viral break though.
A second oral presentation at CROI 2018 looked at low dosing of EFdA when used as treatment, suggesting daily doses as low as 0.25 mg/day would be effective in daily combinations.  At 0.25 mg, EFdA will have to be coformulated to even see it.
Results showing TAF protects against vaginal exposure
Although preclinical animal data in rectal tissue supported TAF use for PrEP, with the ongoing Discover study already enrolled (comparing oral FTC/TDF to FTC/TAF), the first data on vaginal tissue was only presented at CROI 2018.
Ivana Massud presented results from a two-part study in five pigtailed macaques (median age 12 years) showing in part 1 that a 1.5mg/kg TAF dose that resulted in levels that were higher than matched human exposures in plasma but that closely matched intracellular levels in vaginal and rectal tissue . 
The efficacy study was similar to previous studies using oral FTC/TDF, with six animals dosed 24 hours before and 2 hours after weekly SHIV exposures for 16 weeks. Control animals all become infected (median 5 weeks) compared to only one animal receiving FTC/TAF (at week 2). This showed 82% efficacy (p=0.042)
The animal that became infected had good levels of active metabolite for FTC in PBMCs throughout 16 weeks, but tenofovir levels were largely below the limit of quantification. Although the cause of these low levels was not explained, it showed the FTC alone was not sufficient for protection. No resistance was detected in this animal.
First animal studies of PrEP in penile tissue: oral TDF/FTC and long-acting cabotegravir
Charles Dobard from the US CDC presented the first data (ever) on PrEP levels in penile tissue. 
Although approximately 50% of HIV infections globally are estimated to be from transmission into penile tissue (through the foreskin, glans and urethra), this has never been studied directly in animal or human PrEP studies. However, the effectiveness of PrEP in penile tissue has been proven indirectly in heterosexual studies (TDF-2 and Partners-PrEP).
The model was first validated using the same design as the animal studies that showed oral TDF/FTC protected against vaginal and rectal exposure. Six rhesus macaques were given oral TDF/FTC (24 hours before and 2 hours after) with weekly penile exposure to SHIV, for 12 weeks. All control animals became infected by week 12 (at median 2 weeks), with only one TDF/FTC animal becoming infected (at 7 weeks), showing 92% efficacy (p=0.032).
For the cabotegravir study, six animals were injected with cabotegravir LA (50 mg/kg IM) every month for three months with similar weekly exposures to SHIV for 12 weeks. Weekly inoculation was both directly into the urethra and to the pouch under the foreskin.
The results were also similar. All controls were infected by week 12 (median by two weeks) compared to only one animal in the active CAB-LA (at 12 weeks); matching the 93% efficacy reported in the vaginal and rectal studies (p=0.02).
Very good PK was reported for the protected animals with all protected animals having drug concentrations 4-fold above the protein adjusted IC90. In the animal that became infected drug levels dropped below this level for the fourth week of each cycle.
All eight controls were all infected within 12 weeks, with 50% by 2 weeks, compared to only 1/6 animals in active group (at week 7), producing efficacy of 92% (p=0.032).
Vaginal insert for PrEP against HIV, HSV-2 and HPV: griffithsin/carrageenan (GRFT/CG)
Another animal study included results from using a fast-dissolving vaginal implant design to protect against HIV, HSV-2 and HPV. 
GRFT is a small lectin derived from red algae that blocks HIV entry without having cross-resistance to current ARVs and that also has activity against HSV and HPV. This study used a version grown from the tobacco plant. CG is a polysaccharide also derived from algae that has potent activity against HPV.
In macaque studies, the fast-dissolving combination insert (developed with PATH in Seattle) produced GRFT target levels 100-fold above the EC90 in cervical vaginal lavage within an hour that were sustained over 24 hours for most animals. The macaque studies also looked at the impact of pretreatment with depot medroxyprogesterone acetate (DMPA) long-acting progestin contraceptive.
Ten animals in each group were challenged by SHIV four hours after the insert was applied. All control animals, using only CG, became infected compared to only 2/10 of the GRFT/CG active group (p=0.003). There were no differences in viral dynamics or immunologic responses between infected and control animals.
Activity against HSV-2 was shown in a mouse study. All 15/15 mice become rapidly infected, showing symptoms within 7 days compared to only 6/15 in the active arm – leaving 60% of the mice receiving GRFT/CG disease-free, (p<0.0001).
HPV protections was shown in mice challenged with HPV-16, with all control animals showing evidence of infection compared to active group (p<00001).
On the basis of these results, the first phase 1 human study is already underway, using a GRFT/CG gel.
Separate phase 2 studies are also underway using CG gel to prevent HPV.
Long-acting PrEP using bNAbs
There is sufficient potential for long-acting broadly neutralising antibodies to be used as PrEP that two large phase 2b studies are already ongoing using VRC01.
A preclinical animal study at CROI 2018 looked at using two different gp120-binding bNAbs – 3BNC117 and 10-1074 – that protected against rectal exposure in animal studies. Both antibodies are more potent than VRC01, though with less broad neutralisation. The new study, presented by David Garber from the US CDC, looked at vaginal protection in three groups of rhesus macaques: (i) 3BNC117 alone (n=6), (ii) BNC117 combined with 10-1074 (n=6) or (iii) a placebo control arm (n=3), followed by weekly SHIV challenges. 
All animals did become infected but at significantly different median times with: 2 weeks for the control group, 5 weeks for the single 3BNC117 antibody arm (p=0.002 vs control) and 11.5 weeks for the dual antibody arm (p=0.002 vs control and p=0.0005 vs the mono arm).
The great protections in the dual arm were related to longer persistence of 10-1074 and higher maximum levels after dosing (rather than a longer half-life).
Neither of the antibody groups showed different viral or immunological responses after infection, compared to the control group.
A separate poster was also presented from a phase 1 study using a combination of two different antibodies that were formulated as a vaginal film. 
Unless stated otherwise, all references are to the Programme and Abstracts of the 25th Conference on Retroviruses and Opportunistic Infections (CROI 2018), 4–7 March 2018, Boston.
- Markowitz M et al. Low dose MK-8591 protects rhesus macaques against rectal SHIV infection. CROI 2018, Boston. Oral abstract 89LB.
- Markowitz M et al. Weekly oral MK-8591 protects male rhesus macaques against repeated low dose intrarectal challenge with SHIVC109P3. IAS 2017, Paris. Late breaker oral abstract MOAX0203LB.
- Matthews RP et al. Multiple daily doses of MK-8591 as low as 0.25 mg are expected to suppress HIV. CROI 2018, Boston. Oral abstract 26.
- Massud I et al. Oral FTC/TAF combination prevents vaginal SHIV infection in pigtail macaques. CROI 2018, Boston. Oral abstract 85.
- Dobard C et al. Long-acting cabotegravir protects macaques against repeated penile SHIV exposures. CROI 2018, Boston. Oral abstract 83.
- Derby N et al. Griffithsin/carrageenan inserts prevent SHIV, HSV-2, and HPV infections in vivo. CROI 2018, Boston. Oral abstract 84.
- Garber DA et al. Protection against repeated vaginal SHIV challenges by bNABb 3BNC117 and 10-1074. CROI 2018, Boston. Oral abstract 82.
- Cu-Uvin S et al. Phase 1 trial to assess safety and antiviral activity of MB66 vaginal film. CROI 2018, Boston. poster abstract 1063.
www.croiconference.org/sessions/phase-1-trial-assess-safety-and-antiviral-activity-mb66-vaginal-film (abstract and poster)