Bailey & Phillips Research Group
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PepT1 Transporter
The transport of intact di- and tripeptides from the apical side of the intestine is mediated by a specific transporter, PepT1. This transporter plays a crucial role in nitrogen metabolism and is responsible for the absorption of the majority of the body’s amino acids (in the form of peptides which are hydrolysed in the cells to release the composite amino acids). A wide variety of drugs are also substrates of this transporter, including penicillins and cephalosporins, ACE inhibitors and nucleoside-based antivirals. PepT1 mediated transport of these drugs accounts for all or a major part of their oral bioavailability.
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Figure 1a - Substrate template for PepT11 |
Figure 1b - Helical wheel plan of PepT12 |
Since PepT1 is a membrane protein with no crystal structure currently available, our efforts to understand this transporter have been twofold: Developing models to predict substrate affinity for PepT1 (Figure 1a) and homology modelling of the PepT1 sequence to that of similar membrane proteins with known crystal structure (Figure 1b).
Improving Oral Bioavailability of Drugs
Our group is currently focused on applying our understanding of this transporter to improve the delivery of drugs. Many interesting lead candidates are shelved at the pre-clinical stage of development due to a lack of oral bioavailability. The oral route is favoured because of its ease of administration and high levels of patient compliance, however it contains significant physical (membranes) and metabolic (gastrointestinal and cytosolic enzymes) barriers to drug absorption. Examples of drugs with interesting therapeutic properties but poor oral activity include gabapentin [1] (anticonvulsant and analgesic) and propofol [2] (migraine and nausea). However, we can imagine the amount of compounds stored by pharmaceutical companies which are not in the public domain, that were shelved at the development stage and could also be amenable to a targeted delivery approach. Additionally, drugs which have high oral activity but also gastrointestinal side-effects (such as ibuprofen [3] and nabumetone [4]) may also benefit from a delivery approach that retains the high bioavailability, but reduces GI toxicity.
Figure 2 - “Drug smuggling” by PepT1
Since the PepT1 transporter has such a broad substrate capacity (consider all possible combinations of di- and tripeptides from proteogenic amino acids), we believed it would readily accommodate a wide variety of drugs IF they could be modified so as to have affinity for the transporter. Our approach utilises a simple “drug smuggling” technique whereby any drug can be attached (by way of a linker if necessary) to a hydrolysis resistant PepT1 substrate “carrier” (Figure 2). The entire prodrug is then recognised as a PepT1 substrate due to the presence of the carrier and is transported from the intestine by the transporter. Metabolism (in the intestinal cells, bloodstream or liver) will eventually release the active drug. Our carrier of choice are proprietary thiodipeptides of Ala-Asp [5] and Ala-Ser [6], which retain high affinity for the PepT1 transporter, but are metabolically more stable in vivo by virtue of their “non-natural” thioamide.
In collaboration with Dr. David Meredith’s group at Oxford Brookes University, we have synthesised thiodipeptide prodrugs [7-10] and tested these in vitro for affinity and transport in PepT1 expressing Xenopus laevis oocytes and additional in a Caco-2 cell monolayer assay with extremely encouraging results. In vivo testing has validated these in vitro results and indicates that this approach is of considerable general merit and applicability.
Targeted Delivery of Cancer Drugs
A variety of cancer cells also express the PepT1 transporter at high levels, presumably due to the increased metabolic demand of these rapidly dividing cells. We are currently investigating the potential of our PepT1 carrier approach to enhance the delivery of cancer drugs such as [11] and [12] specifically to cancer cells expressing PepT1 transporters.
