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Retrocyclin RC-101 Blocks HIV-1 Transmission Across Cervical Mucosa in an Organ Culture

Gupta, Phalguni PhD*; Ratner, Deena BS*; Ding, Ming MD*; Patterson, Bruce MD; Rohan, Lisa C. PhD; Reinhart, Todd A. PhD*; Ayyavoo, Velpandi PhD*; Huang, Xioli MD*; Patton, Dorothy L. PhD§; Ramratnam, Bharat MD; Cole, Alexander M. PhD

JAIDS Journal of Acquired Immune Deficiency Syndromes: 15 August 2012 - Volume 60 - Issue 5 - p 455–461
doi: 10.1097/QAI.0b013e318258b420
Basic and Translational Science

Background: Cervical tissue–based organ cultures have been used successfully to evaluate microbicides for toxicity and antiviral activity. The antimicrobial peptide retrocyclin RC-101 has been shown to have potent anti-HIV activity in cell culture.

Objective: To evaluate RC-101 in organ culture for toxicity and its ability to block HIV-1 transmission across cervical mucosa.

Methods: A cervical tissue–based organ culture was used to measure antiviral activity of RC-101. Cytotoxicity in tissues was determined by immunostaining of cellular proteins and by measuring inflammatory cytokines using real-time reverse transcriptase–polymerase chain reaction and Luminex technology.

Results: RC-101 blocked transmission of both R5 and X4 HIV-1 across cervical mucosa in this organ culture model. Furthermore, film-formulated RC-101 exhibited potent antiviral activity in organ culture. Such antiviral activity of RC-101 was retained in the presence of semen and vaginal fluid. RC-101 showed no cytotoxicity in cervical tissue. Furthermore, RC-101 did not induce proinflammatory cytokine response in tissues. RC-101 also did not have any effect on natural killer cell activity and proliferation of CD4 and CD8 cells and did not show chemotactic activity.

Conclusions: Therefore, because of strong antiviral activity and low cytotoxicity in cervical tissues, RC-101 should be considered as an excellent microbicide candidate against HIV-1.

*Department of Infectious Diseases and Microbiology, School of Public Health, University of Pittsburgh, Pittsburgh, PA

Department of Pathology, Stanford University, Stanford, CA

Magee Women's Research Institute and the Department of Pharmaceutical Sciences, School of Pharmacy, University of Pittsburgh, Pittsburgh, PA

§Department of Obstetrics and Gynecology, University of Washington, Seattle, WA

Division of Infectious Diseases, Department of Medicine, Brown University, Providence, RI

Department of Molecular Biology and Microbiology, Burnett School of Biomedical Sciences, College of Medicine, University of Central Florida, Orlando, FL.

Correspondence to: Phalguni Gupta, PhD, Department of Infectious Diseases and Microbiology, School of Public Health, University of Pittsburgh, 426 Parran Hall, 130 DeSoto Street, Pittsburgh, PA 15261 (e-mail:

Supported by the National Institute of Health Grants R01 HD052436 and NIH U19 AI65430.

None of the authors received honoraria from any company or is on the speaker's bureau or CME organizers for any organization.

The authors have no conflicts of interest to disclose.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's Web site (

Received July 20, 2011

Accepted March 28, 2012

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Despite the decline in HIV-1–related disease and death in the United States over the past 8 years, because of the availability of antiretroviral therapies, there are major gaps in our current HIV-1 prevention strategies. Although an HIV-1 vaccine will be important to the control of the global epidemic, no effective vaccine against HIV-1 is available thus far. In the absence of an effective vaccine against HIV-1, there is an increasing recognition that topical microbicides for vaginal use that can prevent HIV-1 transmission during sexual intercourse may be the most viable current option in controlling the epidemic.1–3 Most of the antiviral properties of microbicides have been evaluated by examining their effect on the replication of virus in human primary T lymphocytes–transformed T-cell lines and epithelial cells.4–9 However, there are concerns that these assays may have less relevance when applied to sexual transmission of HIV-1 across epithelial cells of vaginal and cervical origin.

To address these concerns, a number of investigators have used primary cervical/vaginal tissue–based organ culture and organotypic cervicovaginal tissues to evaluate toxicity and antiviral activity of microbicides.10–18 We have previously established a quantitative high-throughput tissue transmission assay using the organ culture model to evaluate potential topical microbicides for their ability to block HIV-1 transmission across the epithelial mucosal barrier.11,19,20 Compounds evaluated in this system include reverse transcriptase (RT) inhibitors (PMPA and UC781), various HIV entry–inhibiting antimicrobial peptides (WLBU-2, LL37, and UC781), and membrane cholesterol–dissolving drugs, such as beta-cyclodextrin.19,20 In addition, the organ culture assay has allowed us to directly test the potential cellular toxicity of microbicide candidates by measuring immune and nonimmune cellular markers and proinflammatory cytokine responses. Our organ culture assay thus provides an ideal system for evaluating potential topical microbicides in their ability to block HIV-1 transmission across the epithelial mucosa and directly test the potential toxicity of microbicide candidates in cervical tissue.

A limited number of anti-HIV-1 microbicides that can be applied topically in the vagina have been developed. First generation of microbicides, consisting of surfactants and polyanionic polymers that are supposed to inactivate HIV-1 upon contact, were found to be either toxic or ineffective in blocking HIV-1 transmission in clinical trials.21–23 A second generation of HIV-1 RT-inhibiting compounds with potent antiviral activity in vitro has been recently described. Gel formulations of both nucleoside analog PMPA (Tenofovir) and nonnucleoside analogs UC781 and TMC120 are currently in clinical trials.24–27 Vaginal application of 1% Tenofovir gel has recently been shown to reduce HIV-1 transmission by 40%.28 Another class of microbicides in development blocks viral attachment to CD4, co-receptor (CCR5 and CXCR4) interactions, or gp41-mediated fusion. Such compounds may provide a highly effective strategy for preventing localized mucosal infection because they prevent the initial stages of the infectious life cycle.29,30 Unlike surfactant-based microbicides, such an approach is highly unlikely to perturb the protective effects of resident microflora or have contraceptive potential. Cellular co-receptor antagonists, such as CMPD167 and AOP-RANTES (CCR5 inhibitors) and AMD3465 (X4 inhibitor), have been tested in monkeys 30,31 and are being considered potential microbicides in humans.

The cyclic antimicrobial peptide retrocyclin RC-101 is a cationic β-sheet 18-residue peptide that interacts with gp41 and prevents fusion. It is considered a potential microbicide candidate because of its anti-HIV activity against a wide variety of HIV, with no toxicity in cell culture.32–34 In this report, we have evaluated the antiviral and cytotoxic profile of RC-101 in our cervical tissue–based organ culture model. Our data indicate that RC-101 blocks HIV-1 transmission in the cervical tissue–based organ culture. Furthermore, RC-101 showed no cytotoxicity as measured by a variety of immune and nonimmune functions.

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RC-101 Formulation as Intravaginal Films

The 18-amino acid RC-101 peptide was prepared on a 0.25 mmol scale with an ABI 431A peptide synthesizer using FastMoc chemistry. The purified RC-101 was subsequently oxidized and cyclized as described elsewhere.32 The peptides were analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to confirm homogeneity and that the measured mass agrees well with its expected mass. RC-101 was formulated by solvent casting technique as a quick dissolving polymeric 27.5 × 33.5-mm2 vaginal film, composed of 2.0 mg of RC-101, 6% polyvinyl alcohol (Kuraray America, Inc, New York, NY), 0.12% hydroxypropyl methylcellulose 6 (Sigma, St Louis, MO), and 3% glycerin (Dow Chemical Co., Midland, MI) as described by Sassi et al.35 All films were stored in PET/aluminum foil pouches (Amcor Flexibles Healthcare, Inc, Mundelein, IL) until use.

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Virus and Cell Culture

Cell-free HIV-1 BAL was grown in phytohemagglutinin (PHA)-stimulated CD8-depleted PBMCs from seronegative persons and titered in the same cells. CD8-depleted peripheral blood mononuclear cells were prepared from blood bank donors by the use of anti-CD8 monoclonal antibody–coated immunomagnetic beads (Dynal, Oslo, Norway) as described previously.36

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Testing Antiviral Activity of RC-101 in Organ Culture

Cervical tissues were obtained from seronegative premenopausal women aged 50 years or younger undergoing hysterectomy or anterior/posterior repair procedures at the Magee Women's Hospital under an institutional review board–approved protocol of the University of Pittsburgh. The organ culture was set up with CD8-depleted PBMCs (500,000) from a seronegative normal donor as indicator cells in the bottom chamber of the Transwell system as previously described.11,20 A transwell with agarose only in the top chamber served as a negative control, whereas transwells with the membrane only served as a positive control. To measure antiviral activity, cell-free HIV-1 BAL or IIIB (1 × 105 TCID50) were preincubated with RC-101 (10–40 μg/mL) for 1 hour. RC-101 and virus mixture were added to the top chambers of the tissue. As a control, HIV-1 incubated with media for 1 hour was added to another tissue well, and agarose control and membrane control wells, and incubated at 37°C for 3–4 days. After incubation, the top chamber of the well was removed, and culture of CD8-depleted cells in the bottom chamber was continued for an additional 10 days. Viral growth was monitored by measuring HIV-1 p24 antigen levels in the culture supernatant in the bottom chamber of the tissue well. A 3-fold or greater increase in HIV-1 p24 during the 2-week period was taken as positive virus infection of the CD8-depleted cells. On the last day of culture, leakiness of the organ culture system was routinely monitored by examining transmission of blue dextran, a 2 × 106 Da polysaccharide, through the tissue-containing Transwell and the agarose control into the bottom chamber as described previously.19 The amount of blue dextran transmitted was less than 1% in agarose control well, and tissue sample wells routinely showed less blue dextran transmission than did the agarose control wells. Most of the experiments were done with tissues from 2 to 3 subjects in duplicate or triplicates depending on the availability of tissues.

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Measurement on Intracellular Ki67 and Cytokeratin Protein

The level of intracellular Ki67 and cytokeratin (AE1/AE3) was determined by quantitative immunostaining as described previously.11

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Measurement of Proinflammatory Cytokines Response to RC-101

The level of proinflammatory cytokine (IL-1β, IL-6, IL-8, and TNF-α) messages was measured in the tissues by real-time reverse transcriptase–polymerase chain reaction (RT-PCR). Total RNA from tissues was isolated with RNA-Bee (Tel-Test, Inc, Friendswood, TX) and followed by reverse transcription with TaqMan Reverse Transcription Reagents (Applied Biosystems, Carlsbad, CA) according to the manufacturer's protocols. Thirty microliters of PCR mixture consist of 3 μL of cDNA (12 ng of total RNA equivalent), 2× TaqMan Universal PCR Master Mix, and 20× TaqMan Pre-Developed Assay Reagents (Applied Biosystems). ABI Prism 7000 Sequence Detection System was used to carry out real-time PCR under the following cycling condition: 50°C for 2 minutes, 95°C for 10 minutes, 40 cycles of 95°C for 15 seconds, and 60°C for 1 minute. We selected human β2M primers and probe labeled with VIC/TAMRA for endogenous control. Human TNF-α, IL-1β, IL-6, and IL-8 primers and probes labeled with FAM/MGB were used for proinflammatory cytokine measurements. Assays were performed under conditions suggested by the manufacturer, which were designed to exclude the detection of genomic DNA. Each sample was run in duplicate. No template control was applied in each assay to ensure no cross contamination. Relative gene expression data were analyzed with relative quantification (ΔΔCt) method with 7000 System SDS software (Applied Biosystems). Our preliminary data showed that the amplification efficiency of β2M and other cytokines was approximately equal; therefore, using the ΔΔCt calculation was valid in our assay system (User bulletin #2; Applied Biosystems). The cytokine gene expression results were reported as relative fold change. Secreted cytokines in culture supernatant were measured by Luminex technology according to the manufacturer's (Bio-Rad Laboratory, Hercules, CA) instruction.

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Natural Killer Cell–Mediated Target Cell Lysis Assay

PBMCs (5 × 106/mL) from normal donor were stimulated with PHA (1 μg/mL) and IL-2 (200 U/mL) and exposed to the indicated doses of RC-101. Three days posttreatment, cells were washed and degranulation of natural killer (NK) cells within PBMCs was measured after coincubation of total PBMCs with K562 (PBMC/K562 ratio = 1:1 in a total volume of 1 mL) for 2 hours, including a last 1 hour with Golgy stop. Surface staining was performed using CD3-ECD (electron coupled dye/PE-Texas Red), CD56-phycoerythrin (PE), and CD107a-Fluorescein isothiocyanate (FITC) conjugated antibodies. Expression of CD107a in CD3/C56+ gated NK cells was determined by flow cytometry. As a vehicle control cells were treated with 20 μL of 0.1% acetic acid per ml culture volume. Number in quadrant represents percentage of CD3− CD56+ cells expressing CD107a.37

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Cell Proliferation Assay

Cell proliferation assay was done as described previously.38 Briefly, 1 × 105 PBMCs were incubated with carboxyfluorescein succinimidyl ester dye (10 μg of CFSE in 1 mL of 0.2% bovine serum albumin–phosphate-buffered saline; Molecular Probes, Eugene, OR) at 37°C for 10 minutes. After incubation, 5 mL of cold complete medium was added and incubated on ice for 5 minutes. Cells were washed with cold RPMI 1640 medium containing 10% heat-inactivated human AB+ serum (Sigma), 1% L-glutamine, 1% HEPES buffer, and 1% penicillin–streptomycin and cultured for 6 days in various concentration of RC-101 (10–40 μg/mL) at 37°C. After 6 days of incubation, the cells were harvested, washed, and stained for surface markers by using anti-CD8-peridinin chlorophyll protein and anti-CD4-phycoerythrin monoclonal antibody (BD Biosciences, San Jose, CA). After staining, the cells were washed, fixed, and analyzed in a FACS Canto II flow cytometer (BD Immunocytometry Systems, San Diego, CA). Negative controls (medium only) and positive controls (phytohemagglutinin, 5 μg/mL; Sigma) were included in each assay. The results are expressed as net percentage of CFSE-positive T cells (percentage of positive peptide-stimulated T cells − percentage of medium control).

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Chemotaxis Assay

Chemotaxis assays were performed as described previously.39 Briefly, PBMCs were loaded onto top well of a 96-well ChemoTx Chemotaxis System (5-m pore; NeuroProbe, Gaithersburg, MD). RC-101 or the control chemokines were added in the bottom chamber. Cells were incubated for 5 hours at 37°C in 5% CO2; the cells on top of the membrane were removed with a scraper, and the migrated cells in the bottom wells were counted using a hemocytometer.

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Antiviral Activity of Unformulated and Formulated RC-101 in Organ Culture

RC-101 was solubilized in water and tested in organ culture for its ability to block HIV-1 transmission across cervical mucosa. As shown in Figure 1, RC-101 blocked transmission of HIV-1 in a dose-dependent manner. Antiviral activity was demonstrated against both R5 HIV-1 BAL and X4 HIV-1 IIIB. More than 90% inhibition was obtained at 40 μg/mL of RC-101. Antiviral activity was similar to that observed with an RT-inhibiting microbicide UC781.19,20 We also examined the antiviral activity of RC-101 against 2 other clades of HIV-1 of African origin in organ culture. They were ZA/97/003, clade A, R5 HIV-1 and UG/92/037, clade A, and X4 HIV-1. As shown in Figure 1, RC-101 effectively blocked transmission of both African strains across cervical mucosa, although it showed higher antiviral activity against African isolate ZA/97/003.

Next, we measured antiviral activity of film-formulated RC-101. For this purpose, one film containing 2 mg of RC-101 was dissolved quickly in 1 mL of culture medium and tested at various dilutions in duplicate for antiviral activity in cervical organ culture. As shown in Figure 2, film-formulated RC-101 blocked HIV-1 transmission in a dose dependent manner, with more than 80% inhibition at 200 μg/mL. Interestingly, the placebo film also exhibited some antiviral activity.

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Evaluation of Cytotoxicity of RC-101 in Organ Culture

Subtle microbicide-induced cytotoxicity may not be fully ascertained by physical examination or histological examination of the vaginal/cervical tissues. However, measurements of the level of inflammatory cytokines and cellular markers cytokeratin and Ki67 in response to microbicides in tissues in an organ culture model are excellent approaches to monitor cellular injury and cytotoxicity caused by microbicides. Therefore, we determined the effect of RC-101 on the level of intracellular Ki67 protein, a cell division marker, and cytokeratin, an epithelial cell differentiation marker. Tissues in the organ culture format with no indicator cells in the bottom well were exposed to unformulated RC-101 (40 μg/mL) for 24–72 hours at 37°C. After incubation, tissues were harvested from top well, fixed, and analyzed for Ki67 and cytokeratin (AE1/AE3) by quantitative immunostaining, as described previously.11 As shown in Table 1, the relative percentage of Ki67 and cytokeratin in RC-101-exposed cervical tissues ranged between 20.4 and 26.1 for Ki67 and between 36.9 and 44.5 for cytokeratin. This was similar to levels recovered from controls (17.5%–24.7% for cytokeratin and 38%–41% for cytokeratin). In contrast, treatment with N-9, a known cytotoxic microbicide, resulted in approximately 3-fold reduction of Ki67 (7.3%) and 2-fold reduction in cytokeratin (19.7%) markers.

Next, we measured, in our organ culture, the inflammatory cytokine response to unformulated RC-101. Our standard organ culture was set up as described above, with no indicator cells in the bottom. Cervical tissues were exposed to RC-101 (40 μg/mL) for various lengths of time. Tissues were then harvested and examined for IL-1β, IL-6, IL-8, and TNF-α cytokine messages by real-time RT-PCR and secreted cytokines in the supernatant by the Bio-Rad Bio-Plex System using the Luminex technology according to the manufacturer's (Bio-Rad Laboratory) instruction. The RT-PCR assay conditions were designed to exclude the detection of genomic DNA. Each sample was run in duplicate, and average CT values were used for gene expression calculation. Results are expressed as the fold increase in cytokine expression as compared with tissue incubated in media alone. Tissues treated with RC-101 for 24–48 hours exhibited low level of cytokine messages (see Figure, Supplemental Digital Content 1, and secreted cytokine proteins (Fig. 3) similar to controls. Longer incubation resulted in slightly higher levels of TNF-α protein (Fig. 3). In contrast, N9, a microbicide known to cause epithelial irritation in women,40–42 elicited more than 5 folds of TNF-α messages within 24 hours of exposure of tissue with 4% N9, reaching a maximum level of more than 30 folds with 1% and 4% N9 after 48 hours. A 3-fold increase in IL-1β messages was also noted after 48-hour culture, after exposure to 1% N9 for 1 hour (data not shown). There was no change in the level of expression of IL-6 and IL-8. Loss of inflammatory response at 72 hours in 4% N9 was due to loss of cell viability at 72 hours at that concentration. Because there were no indicator cells in the bottom well, we conclude that cytokine response was elicited by the tissues.

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Effect of RC-101 on Immune Functions

Cervical tissues exposed to HIV-1 induce a number of immune activators to control viral infection. We evaluated the effect of unformulated RC-101 on the following 3 common immune parameters: NK cell activity, chemotactic activity, and cell proliferation. Because these experiments are difficult to perform in tissues, we evaluated effects of RC-101 in lymphocyte cell cultures. It is noteworthy that RC-101 also exhibits profound antiviral activity in PBMC (P. Gupta, manuscript in preparation). As shown in Supplemental Digital Content 2 (see Figure,, RC-101 up to 20 μg/mL had no significant effect on NK cell activity, although it showed some inhibition at 40 μg/mL. Similarly, RC-101 up to a concentration of 40 μg/mL had no significant chemotactic activity of lymphocytes compared with positive control chemokines CXCL11 and CCL21 (see Figure, Supplemental Digital Content 3, RC-101 up to a concentration of 40 μg/mL also showed little effect on proliferation of CD4 and CD8 lymphocytes. As a positive control, PHA showed high-degree proliferation (see Figure, Supplemental Digital Content 4,

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Effect of Seminal and Vaginal Fluids on the Antiviral Activity of RC-101 in Cervical Organ Culture

The antiviral activity of RC-101 was evaluated in the presence of 10% human seminal and vaginal fluids. For this purpose, unformulated RC-101 was incubated for 1 hour at 37°C with HIV-1 BAL, in the presence and the absence of 10% seminal fluid or vaginal fluid obtained from seronegative control men and women, respectively. Because of the reported toxicity of seminal fluid on cells/tissues, we have recovered HIV-1 after incubation from seminal fluid or by high-speed centrifugation of the semen/microbicide mixture. Pelleted virus was resuspended in small volume of medium and then added onto the tissue and cultured for 3–4 days. Transmitted virus was measured by its growth in the indicator cells in the bottom well. Because vaginal fluid is not toxic for cells, after incubation, we added vaginal fluid/HIV-1/RC-101 mixture directly into cervical tissue in the organ culture. As shown in Figure 4, seminal fluid or vaginal fluid did not significantly affect antiviral activity of RC-101 in cervical tissues.

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Retrocyclins are members of a class of antimicrobial peptides called θ-defensins. Although monkeys produce θ-defensin peptides, humans cannot produce these peptides because the gene contains a premature stop codon in the peptide's signal sequence.43 Previous studies have demonstrated that retrocyclins, such as RC-101, can protect primary T cells from in vitro infection by both X4 and R5 strains of HIV-1 and are much more active in vitro than other closely related defensin molecules.32–34 However, the relevance of these assays is uncertain when considered in the context of sexual transmission of HIV across epithelial cells of vaginal and cervical origin. Therefore, in this report, we evaluated RC-101 in a cervical tissue matrix in an organ culture that closely mimics in vivo conditions. We have shown that RC-101 blocks transmission of both R5 and X4 HIV-1 across cervical mucosa in this organ culture model. Furthermore, film-formulated RC-101 retained antiviral activity in organ culture. However, it needed higher concentration of RC-101 when film formulated to achieve same level of suppression by the unformulated RC-101. This is not unexpected because similar situation was reported for other microbicides, for example, 1% RC-101 produces same amount of antiviral activity as 0.01% RC-101 in aqueous form.35 Both vaginal and seminal fluids had no deleterious effect on the antiviral activity of RC-101 in organ culture, although seminal plasma itself has some antiviral activity, which is not due to toxic effect. It is possible that semen has innate immunity against HIV-1. Regardless, these data further illustrate the importance of RC-101 as microbicide by demonstration of its antiviral activity in the presence of semen and vaginal fluids.

Because microbicides will be topically applied to the vagina, it is important to determine their cytotoxicity in vaginal/cervical tissues. The failure of N9 and cellulose phosphate in phase 3 clinical trial21,22,44 warned us that safety evaluation of a microbicide candidate should be performed as early as possible. Although monitoring for lesions in the cervix/vagina has been used to test for cytotoxicity in phase 1 clinical trials for microbicides,21,22,45 it cannot detect subtle changes in the mucosal barrier, such as induction of clinically inapparent inflammation. Although proinflammatory responses are beneficial to control vaginal bacterial infection, inflammatory cytokines may enhance HIV-1 transcription in infected cells and increase HIV-1 transmission.46,47 The rabbit vaginal irritation model has also been used to study the inflammation and toxicity of the drug formulation to the genital mucosa. Its usage is limited because of anatomical differences of the vaginal and cervical mucosa between human and rabbit. The cervical tissues in our organ culture conditions have been shown to maintain histological and cellular (immunological and nonimmunological) markers during the 6 days of incubation period 11 and therefore provide a relevant and convenient model to evaluate microbicides for potential cytotoxicity in the tissue matrix. Using this organ culture, we have demonstrated that RC-101 had no cytotoxic effect as evidenced by lack of any inhibition on 2 key cellular proteins Ki67, a cell proliferation marker, and cytokeratin, an epithelial cell marker. Furthermore, RC-101 did not induce proinflammatory response in tissues up to 72 hours after exposure. RC-101 also did not alter key immune functions such as NK cell activity and cell proliferation of CD4 and CD8 cells and did not show chemotactic activity of lymphocytes. These results together with previously reported in vitro data showing its nonhemagglutinating properties32 strongly suggest that RC-101 would be a safe microbicide for application in human. This suggestion gained support from our recent collaborative study in the nonhuman primate model showing no vaginal cytotoxicity.48

Currently, microbicides based on nucleoside RT inhibitors (UC781 and TMC120) and one with a nucleoside RT inhibitor (Tenofovir) are in Phase 2 and 3 clinical trials. However, there is a definite need to identify new antiviral compounds as backup microbicidal agents because many drugs with promising preclinical properties fail during advanced clinical evaluation, as they may be ineffective in preventing sexual transmission of resistant variants. In that regard, RC-101 induces very low level of resistance even after 28 passages in cell culture49 and that can be overcome with slight increases in peptide concentration (A.M.C., unpublished data). Retrocyclin being an evolutionary conserved host protein32,50 may be responsible for induction of low resistance. An HIV-1 entry–inhibiting microbicide, such as RC-101, has distinct advantage over RT-inhibiting microbicides, in that it blocks HIV-1 transmission before the virus can infect a target cell. Therefore, probability of developing resistance is lower because of limited viral replication at the beginning of infection.

In summary, RC-101 possesses many of the properties of an ideal microbicide candidate with strong antiviral activity and low cytotoxicity in cell culture and tissues. For these reasons, combined with recent safety demonstrated in the macaque model,48 RC-101 should be considered an excellent microbicide candidate for clinical trials.

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The authors thank Ms Varsha Sridhar and Dr Yue Chen for editorial assistance and Dr Yongjun Sui for technical assistance.

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retrocyclin; HIV; organ culture; microbicide

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