MB66

MB66 is an investigational vaginal film carrying two plant-produced monoclonal antibodies. HSV8-N is directed against herpes simplex virus and VRC01-N against HIV-1.

It reached one completed Phase 1 study in 38 healthy women, which ran from 2016 to 2018. That study reported no serious adverse event, antibody levels in vaginal fluid well above laboratory inhibitory concentrations, and vaginal fluid that neutralized HSV-2 in laboratory assays.

It did not establish clinical prevention. No acquisition or transmission outcome was measured, for either virus, at any endpoint level.

No later study of MB66 was identified through September 8, 2026, and no source states that development stopped. MB66 has never been approved or offered outside that one study in the checked United States routes.

MB66 is a small polyvinyl alcohol film for vaginal insertion. It carries 10 milligrams each of two human monoclonal antibodies, and the matching placebo film uses the same ingredients without them.

HSV8-N is the anti-HSV component and VRC01-N is the anti-HIV-1 component. An HIV finding in this program is therefore not an HSV finding. Both antibodies were produced in a plant, Nicotiana benthamiana, which is what the letter N in each name denotes.

The primary publication describes the HSV antibody as binding a protein shared by HSV-1 and HSV-2, and as neutralizing both in laboratory work. The trial registry names no HSV type. The only virus used in the trial itself was HSV-2, so the human evidence covers HSV-2 alone.

The study tested whether the film was safe and whether its antibodies reached the vaginal compartment. Its prespecified primary endpoint was the rate of moderate or worse adverse events judged related to the product.

No efficacy, acquisition, transmission, shedding or lesion endpoint was prespecified at any level. The study enrolled healthy women at low risk of infection who agreed to abstain from vaginal intercourse across dosing, a design that could not have observed an infection outcome.

The single study, registered as NCT02579083, enrolled 38 participants at one hospital in Rhode Island between January 2016 and July 2018.

In the first segment, a clinician inserted one film in nine participants. The second segment was randomized and placebo-controlled: 29 participants inserted one film daily for seven days, 15 assigned to MB66 and 14 to placebo.

Every active arm in both segments delivered the same combination film. No arm gave the anti-HSV antibody by itself.

The results were published in 2021. No results had been posted to the trial registry as of September 8, 2026.

No serious adverse event was recorded in either segment. Moderate or worse events judged related to the product occurred in 2 of 15 MB66 recipients and 1 of 14 placebo recipients, at p=1.00.

Antibody concentrations in vaginal samples stayed above baseline 24 hours after both the first and the seventh film. Corrected for sample dilution, the anti-HSV antibody ranged from 80 to 601 micrograms per milliliter at that point, against a reported inhibitory concentration of 0.1.

In exploratory laboratory assays, vaginal samples taken 24 hours after film use neutralized HSV-2 relative to baseline. Samples taken seven days after the last film did not.

Not every comparison reached significance. In the first segment the overall test across timepoints gave p=0.052, and in the second segment the test for an effect of antibody concentration gave p=0.056. These were exploratory measures in a study not designed to test them.

Some baseline and placebo samples also neutralized virus. The authors attribute that to participants' own antibodies and to other vaginal defenses.

A 2020 review co-authored by several of the trial's own investigators described the completed testing in efficacy terms. That characterization exceeds what the checked human evidence establishes, and the investigators' own trial report states that efficacy remains to be evaluated.

The most defensible evidence-maturity label is early human evidence. MB66 has been given to people in a completed Phase 1 study of safety, antibody concentrations and laboratory activity, and nothing in the checked record measures clinical benefit.

The single most important limit on this record is that the study established delivery and short-term safety, not prevention. Neutralization of virus by a participant's vaginal fluid in a laboratory dish is antibody activity in a sample, not protection of a person.

A concentration above a laboratory inhibitory concentration supports the delivery concept. It does not define a protective threshold in anyone.

The mouse protection experiment cited as the antibody's preclinical basis is animal evidence. It does not establish human protection. As of September 8, 2026, no approved product containing these antibodies was identified in the checked United States routes.

HSVHQ records MB66 as status unclear rather than discontinued. Its only human study completed in 2018 and was reported in 2021, no later study was identified through September 8, 2026, and no source states that development stopped. Absence from a current company pipeline page is not evidence that a program ended.

The trial registry's lead sponsor changed from LeafBio to ZabBio in 2021, and to KBio in 2023. A field-by-field comparison of the stored registry versions shows that the 2023 change altered the sponsor name and some descriptive text, and left status, phase, enrollment, arms, eligibility and outcome measures unchanged. It is an administrative rename, not a status change.

The anti-HSV antibody is separately the subject of a 2025 federal award for preclinical development against neonatal herpes, running to 2027. That is a different indication, a different population, and a systemic route rather than a vaginal film. It does not mean that MB66 is continuing.

The next result that would change this record is a registered and initiated later-phase study of MB66 itself, or a statement from a current rights holder about whether the product is being pursued.

Progress in the separate neonatal-herpes program would change the record for the antibody, not for MB66. The milestone to watch there is a submitted investigational new drug application rather than another funded plan.