Alfred Schinkel
Our research focuses on genes and proteins that affect drug
resistance or drug susceptibility in tumors, or influence the
pharmacological and toxicological behavior of anticancer and other
drugs and toxins, including carcinogens, in mice and man. Insight
into these systems may:
- Improve chemotherapy/pharmacotherapy approaches for cancer and
other diseases
- Increase understanding of risks and opportunities of drug-drug
interactions mediated through these systems
- Increase insight into factors determining susceptibility to
toxins and carcinogens
- Allow elucidation of physiological functions
To study the roles of the proteins involved, and their
interactions, we generate and analyze knockout and transgenic mice
for the relevant genes as well as appropriate in vitro systems.
Most of the studied proteins can be profoundly inhibited or induced
by pharmacological modulators. This affords a window of opportunity
to modulate these systems to improve pharmacotherapy, but it also
involves the risk of adverse drug-drug interactions.
Active drug efflux transporters
Plasma membrane proteins of the ATP binding cassette (ABC)
multidrug transporter family, including P-glycoprotein (P-gp,
ABCB1), MRP2 (ABCC2) and BCRP (ABCG2), can actively export a wide
range of anticancer and other drugs from cells. These proteins can
cause multidrug resistance in tumor cells, and significantly
influence the pharmacokinetics of many drugs, including oral
availability and tissue penetration. Interindividual variation in
activity of these proteins can occur due to genetic polymorphisms,
and due to extensive inhibition with various compounds, which can
dramatically affect toxicity and therapeutic application of drugs.
Analysis of P-gp, Bcrp and Mrp2 knockout mice generated by us has
yielded a wealth of information on these functions, and some
insights have resulted in ongoing clinical trials in patients to
optimize oral pharmacokinetics of anticancer drugs (collaboration
with Schellens and Beijnen). We are currently studying overlapping
physiological, pharmacological and toxicological functions of these
proteins in various compound knockout mouse strains.
Major discoveries we made:
- P-gp and BCRP are essential elements of the blood-brain
barrier, and keep many drugs out of the brain by pumping them back
into the blood. They therefore are major determinants of the
clinical use of many drugs, as they avoid their central nervous
system (brain) effects. On the other hand, this activity may reduce
efficacy of anticancer drugs against brain tumors or
micrometastases partly protected by the blood-brain
barrier.
- P-gp and BCRP can be important factors in limiting the oral
availability of many drugs and toxins, as in the intestinal wall
they can pump these compounds back into the intestinal lumen. This
function limits the clinical use of many drugs, but it also
sometimes provides important protection from natural (dietary)
toxins.
- BCRP is an important transporter in the lactating breast, where
it transports and even concentrates many drugs, carcinogens and
other xenotoxins, but also vitamin B2 (riboflavin) into the milk.
It thus contributes to the risk of exposing suckling infants and
young to drug and pesticide residues.
- BCRP and P-gp expression in the placenta protects the unborn
fetus from exposure to drugs, pesticides and toxins present in the
bloodstream of the mother.
Drug-metabolizing enzymes
Arguably the most important factor for variable drug exposure is
the Cytochrome P450 3A system (CYP3A), which metabolizes >50% of
drugs. CYP3A activity can vary dramatically due to inhibition or
induction by co-administered drugs or food components, and is
therefore a major factor in drug-drug and drug-food interactions.
To assess the in vivo impact of CYP3A, we have generated complete
Cyp3a knockout mice, and mice with transgenic overexpression of
CYP3A4, the primary human CYP3A enzyme. Analysis of these mice
provides insight into the tissue-specific contribution of (human)
CYP3A to variable drug exposure and low oral availability of drugs.
These insights can be of great value to improve drug administration
regimens, but also during the development of optimally efficacious
drugs, for cancer and other diseases. We are currently focusing on
using the insights obtained to improve the oral bioavailability of
taxanes, and testing co-administration regimens to support
analogous efforts in patients.
Important discoveries we made:
- Intestinal CYP3A activity can be far more important than
hepatic CYP3A activity in limiting the oral availability of
substrate drugs.
- P-gp and CYP3A in the intestine can collaborate efficiently in
limiting the oral availability of shared substrate drugs, but they
do not have an obvious synergistic mechanism of action in doing
so.
Drug uptake transporters
Many drugs need transport proteins in order to be efficiently
taken up into cells. This can affect pharmacokinetically important
characteristics, such as uptake of drugs from the intestinal lumen,
uptake into the liver or many other tissues, and uptake into tumor
cells. Hence, variable activity of the uptake transporters can
affect oral availability, elimination rate and route, tissue and
tumor distribution of drugs, and therefore therapeutic efficacy and
risks of toxic side effects. We generated knockout mice for the
organic cation transporter family (Oct/Slc22a1-2), and established
its impact on tissue distribution and elimination of anticancer
drugs and model compounds. More recently, the importance for drug
disposition of multidrug uptake transporters of the organic
anion-transporting polypeptide family (OATP/SLCO) has been studied
through the generation and characterization of knockout and
transgenic mice for the SLCO1A/1B family. We are currently
extending this work to additional OATP drug uptake family
members.
Major discoveries we made:
- The human Rotor syndrome, a previously unexplained hereditary
conjugated hyperbilirubinemia disorder, is caused by a simultaneous
and complete genetic deficiency in two main liver uptake
transporters, OATP1B1 and OATP1B3.
- Efficient detoxification of bilirubin glucuronide by the liver
requires functioning of an extrusion-reuptake loop across the
basolateral membrane of hepatocytes ("hepatocyte
hopping").
Clinical translation
Collaborations with the groups of J.H. Beijnen (hospital
pharmacist), J.H.M. Schellens (clinical pharmacologist and doctor
of internal medicine) and O. van Tellingen at the NKI-AVL allow
rapid translation of obtained insights into clinical trials in
cancer patients.