Stand and deliver: why drug transport is more important than you think…

Treatment options for cancer are continuing to improve, with a plethora of new targeted drugs now licenced for use as part of the therapeutic strategy across a wide range of cancer types. Given the heterogeneous nature of cancer, these drugs have allowed us to begin to use the genetics of a patient’s tumour to devise a treatment plan that is more personalised to them, and not just a stock response to their broader diagnosis.

While undoubtedly representing a beacon of hope, how exactly we personalise targeted therapies has yet to be perfected, and there is still significant variation in patient treatment response.

This is exemplified by a newly-licenced KRas inhibitor, sotorasib. Sotorasib specifically inhibits the G12C mutant form of the KRas protein, irreversibly binding the cysteine at position 12. However, despite this clear genetic biomarker with which to stratify patients, less than 40 percent of patients showed an objective response in phase II clinical trials. And as with all targeted therapies, of those patients that do respond, sadly drug resistance will, in many cases, limit the utility of the drug far sooner than we would hope.

There are a multitude of factors that determine patient therapeutic response and resistance, and not all of them can be genetically explained. One detail that is often overlooked, is how exactly these drugs enter their target cells, and how this impacts the intracellular concentration that the drug is able to reach.

Classically, intracellular concentration has been assumed to be dependent on the physicochemical properties of the drug in question, with lipophilicity being a major component of the ‘Lipinski’s rule of 5’. It is this which has provided medicinal chemists with a rule of thumb for drug design. Evidence suggests however, that rather than passing directly through the lipid bilayer, many drugs in fact enter cells via membrane transport proteins. For those drugs that do require transporters for import, the expression of that transporter will determine the intracellular concentration reached. And, of course, that concentration is vital for the drug’s function: too low and the intended target will not be engaged, while at high levels, other lower affinity targets may be bound, potentially leading to a poly-pharmacological effect in the tumour, or even toxicity in non-target tissues.

Knowledge gap

There are two major families of membrane transporters; the solute carriers (SLCs) and the ABC efflux pumps, which comprise nearly 500 genes between them.

In the most part, these transporters exist to facilitate the uptake and efflux of nutrients and waste products, and to control the passage of metabolites into and out of different membrane-bound compartments within the cell. Given the heterogeneity of cancer metabolism, both at the inter-patient and intra-tumoural level, it is perhaps not surprising that these important metabolic regulators are also heterogeneously expressed.

For drugs that cross the membrane via transporters, the expression level of these proteins could be a factor in how a patient responds. To be able to use these as functional biomarkers however, we need systematic approaches to close the knowledge gap as to how each cancer therapeutic drug enters cells to reach its target protein.

A subset of broad specificity SLC transporter families, as well as ABC transporters such as ABCB1 (P-glycoprotein) and ABCG2 (BCRP), are well studied for their influence in pharmacokinetics and pharmacodynamics and are regularly screened for in drug development. Emerging evidence suggests, however, that other transporter families may also play roles in determining intracellular drug concentrations, and significant debate continues within the field as to the proportion of drugs that enter via the lipid bilayer versus those that require transporters.

Heterogeneity of transporters

With support from CRUK and the MRC, my lab aims to understand the roles of transporters in drug uptake within tumours, and how this intersects with the role that they play in cancer metabolism.

To do this, we are using a range of systems-level approaches, in combination with cell and patient-derived models. We want to establish whether we can use the transporter expression profile of a patient’s tumour to help us to predict which drugs they are likely to respond to.

As well as inter-patient differences, it is also important to consider intra-tumour heterogeneity when it comes to transporters. The specifics of the local metabolic environment of tumour cells – be that proximity to blood vessels or the local density of stromal or immune cells – will not only influence their transporter expression but will also determine the local levels of potentially competing transporter substrates within the interstitial fluid.

By understanding how these transporters contribute to the tumours’ metabolism, we hope to gain knowledge that could provide us with rational approaches to maximise transporter expression, minimise competing substrates and therefore improve drug permeability in tumours. Equally, if providing a means for drug entry into cells, it is possible that down regulation of a transporter could be a contributing factor to drug resistance.

Increasing public availability of well-annotated RNA sequencing data that includes both pre- and post- treatment sampling of tumours will allow us to further investigate that, as well as a host of other non-genetic mechanisms of resistance.

Personalisation of patient therapeutic plans is beginning to change the way we treat cancer and improving outcomes. Going forwards, as the non-genetic as well as genetic factors of a patient’s disease start to be considered, we will be able to push this personalisation further.

We hope that being able to predict drug permeability will become part of a toolkit to enable the drugs with the best chance of working to be chosen for each patient.