Multicompartmental Pharmacokinetic Model of Tenofovir Delivery to the Rectal Mucosa by an Enema.
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2017
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Abstract
Rectal enemas that contain prophylactic levels of anti-HIV microbicides such as tenofovir have emerged as a promising dosage form to prevent sexually transmitted HIV infections. The enema vehicle is promising due to its likely ability to deliver a large amount of drug along the length of the rectal canal. Computational models of microbicide drug delivery by enemas can help their design process by determining key factors governing drug transport and, more specifically, the time history and degree of protection. They can also inform interpretations of experimental pharmacokinetic measures such as drug concentrations in biopsies. The present work begins rectal microbicide PK modeling, for enema vehicles. Results here show that a paramount factor in drug transport is the time of enema retention; direct connectivity between enema fluid and the fluid within rectal crypts is also important. Computations of the percentage of stromal volume protected by a single enema dose indicate that even with only a minute of enema retention, protection of 100% can be achieved after around 14 minutes post dose. Concentrations in biopsies are dependent on biopsy thickness; and control and/or knowledge of thickness could improve accuracy and decrease variability in biopsy measurements. Results here provide evidence that enemas are a promising dosage form for rectal microbicide delivery, and offer insights into their rational design.
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Katz, DF, and Y Gao (2017). Multicompartmental Pharmacokinetic Model of Tenofovir Delivery to the Rectal Mucosa by an Enema. PLoS One, 12(1). p. e0167696. 10.1371/journal.pone.0167696 Retrieved from https://hdl.handle.net/10161/13493.
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David F. Katz
Dr. Katz's research interests emphasize methods for prophylaxis against STD's, including topical microbicides, and for contraception. Core perspectives and approaches include mass transport phenomena and biofluid mechanics; rheology and imaging, both in vitro and in vivo. These are integrated to fundamental biological, pathological, and clinical processes and needs for improvement.
Context. According to the Joint United Nations Programme on HIV/AIDS, 40 million people worldwide are currently living with AIDS. Even 13 years ago, in 2003, 3 million people died of AIDS and 5 million new people became infected. These epidemiological figures are not significantly improving. Prevention of initial infection by HIV, as well as treatment of the AIDS disease, is critical to combating this terrible worldwide pandemic. Vaccines are one type of AIDS prevention modality, and there is much current research on them. However, this research is expected to require about another 6 - 10 years before efficacious products might become available, and there is no guarantee that successful vaccines will be developed, even in this extended time frame. An alternative prevention method is the use of topical formulations (e.g. gels, foams, intravaginal rings, suppositories, etc.) that a woman inserts into her reproductive tract and which prevent HIV (or other pathogens) from initiating infection. Some of these can also be applied rectally to prevent infection in that compartment. Academic and industrial researchers are currently developing and evaluating a new class of compounds, termed topical microbicides. These compounds kill or dysfunctionalize HIV or other target pathogens upon contact in the female reproductive tract. To-date, microbicide research has been directed almost exclusively toward discovery of new, improved microbicidal compounds. There has been virtually no companion effort on rational design of the formulations which women apply and which deliver microbicides to the target pathogens. We do not yet understand what physical and chemical properties microbicidal formulations should have in order to be most effective in preventing infection. Objective standards for evaluation of such formulations do not exist. The effectiveness of microbicidal formulations depends upon their ability to coat the surfaces of the lower reproductive tract, and to remain in place during the time when a woman is exposed to HIV. Our research group is pioneering analysis of the physical and chemical mechanisms responsible for such distribution and retention of microbicidal formulations (termed deployment) and for the delivery of the microbicidal molecules to target tissues, fluids and the pathogens themselves. This work is being translated to practical application in the evaluation of current candidate microbicidal formulations and in development of improved ones.Our work also focuses upon methods of contraception, drawing upon Dr. Katz's vast experience in working in this field and in reproductive biology and medicine more broadly. in particular, we are working on methods, primarily for use by women and under their control, that simultaneously provide contraceptive protection, and protection and HIV and other sexually transmitted pathogens, e.g. human papilloma virus (HPV) and herpes simplex virus (HSV).
Our research on microbicidal formulation deployment and drug delivery utilizes the methods of mechanical, chemical, electrical and optical engineering and materials science. Our work integrates two types of research: (1) we are undertaking fundamental studies of the mechanisms of formulation deployment and delivery; and (2) we are imaging and analyzing the distributions of formulations over the tissue surfaces of the lower reproductive tract in women, and the concentrations and transport of drugs from formulations onto and into luminal fluids and mucosal tissues. The fundamental studies have two components: theoretical analyses of fluid mechanical and other mass transport processes underlying deployment and drug delivery are linked to experimental simulations (in the laboratory) of salient flow and mass transport mechanisms. For example we developed a spectrophotometric method for analyzing microbicide diffusion from a formulation into cervical mucus. We have built experimental simulations of how gravity and the squeezing forces of vaginal epithelial surfaces cause a formulation to flow over those surfaces. We have built a simulation of how mixing of a microbicidal formulation with ambient in vivo fluids (e.g. vaginal fluid, semen) alters the formulation's properties and, therefore, its tendency to flow and adhere to epithelial surfaces. We also perform experimental measurements of fundamental material properties of formulations such as rheological and surface properties. These serve as inputs to the theoretical models of mass transport and flow. The mathematical models reveal particular relationships between properties of formulations and deployment and delivery characteristics.
We also measure in women the spreading and retention in the vagina of different vaginal formulations. Our human studies are conducted in the clinic of the Department of Obstetrics and Gynecology at the Duke Medical Center. We apply a new, unique imaging instrument built by us, that shares some features with the endoscopes currently used clinically to visualize some interior regions of the body. Our instrument measures the local thickness of coating of the vaginal surfaces with a test formulation. It detects 'bare spots' of uncoated tissue that might be particularly vulnerable to infection. We have also pioneered use of optical methods to measure local drug concentrations, as they are transported from their vehicles into luminal fluids and mucosal tissues. This latter work has initially been in vitro. We are currently working to expand it to direct in vivo measurements, which will be a breakthrough technology in analyzing mucosal drug delivery.
The above biophysics and biomedical engineering based work is also integrated to behavioral studies of users' perceptions and preferences of different contraceptive and STD prophylactic products. The goal is to improve willingness to use these products, by designing them with properties and dosage regimens that achieve their pharmacological goals simultaneously with their user friendliness.
Collectively, our computational, in vitro and in vivo studies are integrated in creating new perspective and algorithms for product design and performance evaluation. They are currently being applied in development of a number of products in the HIV/STD prevention and contraception pipelines.
Our research has been sponsored by the National Institutes of Health, the US Food and Drug Administration (FDA), the American Foundation for AIDS Research, the US Agency for International Development, the Population Council, and the CONRAD Program.
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