Translational research is all about finding novel therapeutic targets and seeing if they’re relevant to a disease. One of the most challenging (and possibly nerve-wracking) scenarios for a drug-discovery team can be investigating a ‘promising’ new target – the scientific and commercial benefits can be profound. In some circumstances, standard biological target validation methods such as knock-out mice, antibodies, etc. which could give confidence in the concept might not be possible. In which case the future of the project may well rely on getting chemical validation in an animal.
To a medicinal chemist this sort of programme will look very different to your standard me-too-we-want-a-slice-of-the-pie-type drug discovery project where the expected outcome is an exquisitely drug-like and squeaky clean molecule ready for the clinic (ideally!). The emphasis will be more on quickly finding a ‘tool compound’ that ‘finds and binds’ your new target so you can see if it has the desired effect in an in-vivo disease model. Finding a compound which binds is usually the easier part, but because increasingly more challenging targets [i.e. with fussy active sites] are being explored the search for a tool with great binding AND great pharmacokinetics can be a nightmare [I’m thinking getting Bcl-2 inhibitors into the brain, for instance]. You now have a couple of options – use skill and tenacity (and time and money) to optimise your compound’s PK, or use a pharmacokinetic ‘get out of jail free card’ to help by-pass the most frequently encountered barriers….
- The Gut Wall: For a poorly absorbed compound (low solubility/permeability) the gut can be by-passed by simply injecting a compound (obvious!), which probably results in significantly better exposure. If i.v. injection doesn’t provide sustained drug levels however (perhaps due to high clearance), an osmotic pump1 can be implanted subcutaneously. Osmotic pumps consist of a capsule containing drug and osmogens, coated with a semipermeable membrane. As the core absorbs water, hydrostatic pressure pushes the drug solution out at a controllable rate through the delivery ports – hey presto, sustained delivery!
Figure 1. An osmotic pump allows controllable and sustained compound exposure: http://www.alzet.com/products/ALZET_Pumps/howdoesitwork.html
2. The Liver: The hepatic reductase null mouse (HRN) is a transgenic mouse developed by Cancer Research UK, which has the Por gene knocked-out. Since this gene encodes the reductase that regenerates the entire cytochrome p450 system, these mice have no hepatic p450 activity. If first-pass metabolism is the cause of poor PK, then using HRN mice can markedly increase circulating drug levels. [NB: comparison of circulating drug levels in HRN versus wild-type mice can also indicate whether hepatic clearance is your issue and whether efficacy or toxicity is caused by formation of a metabolite]. A comparable chemical cytochrome knock-out can be achieved by pre-administration with the pan-Cyp inhibitor 1-aminobenzotriazole3.
Figure 2. Mean concentration time profiles of (A) docetaxel, (B) midazolam, and (C) theophylline in HRN and WT mice (from Boggs JW et. al. Mol Pharm. 2014 Mar 3;11(3):1062-8).
3. The Blood-Brain Barrier: There are some really exotic methods under investigation to circumvent the blood-brain barrier [including hyperosmotic blood-brain barrier opening and ‘trojan horse’ methods using antibodies or surface-modified liposomes which hijack endothelial cell transcytotic mechanisms]4. However, the P-glycoprotein, an ATP-driven drug efflux transporter is most commonly responsible for restricting access of experimental compounds into the brain5. Again, transgenic and chemical tools are available which can by-pass this obstacle to improve delivery of intrinsically permeable compounds to CNS targets. For example, inhibition of P-glycoprotein by the inhibitor Valspodar increases taxol levels in the brain by ten-fold allowing efficacy against glioblastoma6. However, Elacridar is a more commonly used tool to inactivate P-gp at the intestinal or blood-brain barrier, since it has better PK.
Recently Cornell researchers showed that transiently reducing expression of P-gp with the approved drug Lexiscan can open the BBB for a brief window of time – long enough to deliver various therapies to the brain.7 Transgenic knock-out mice [i.e. mdr1a/1b (-/-)] are also available which do not express functional P-gp and these can be used to engage CNS targets with otherwise non CNS penetrant tool molecules.8
Any one of these tools may just give your compound wings and help you better investigate your novel target.
Blog written by Stephen Brand
- Osmotic Drug Delivery System as a Part of Modified Release Dosage Form. ISRN Pharm. 2012, 528079. doi: 10.5402/2012/528079.
- Inactivation of the hepatic cytochrome P450 system by conditional deletion of hepatic cytochrome P450 reductase. J Biol Chem 2003; 278: 13480–6. doi: 10.1074/jbc.M212087200
- In vitro and in vivo characterization of CYP inhibition by 1-aminobenzotriazole in rats. Biopharmaceutics & Drug Disposition. 2016, 37, 1099. doi: 10.1002/bdd.2000.
- A review of nanocarrier-based CNS delivery systems. Curr Drug Deliv. 2006;3:219–232.
- Blood-brain barrier drug targeting: the future of brain drug development. Mol Interv. 2003; 3: 90–105. doi: 10.1124/mi.3.2.90.
- Transport of paclitaxel (Taxol) across the blood-brain barrier in vitro and in vivo. J Clin Invest 2002;110:1309–1318. doi: 10.1172/JCI15451.
- Normal viability and altered pharmacokinetics in mice lacking mdr1-type (drug-transporting) P-glycoproteins. Proc Natl Acad Sci U S A. 1997, 94(8), 4028-33.