A Forum for Improving Drug Safety

COMMENT: This article helps develop a picture of hepatic cells transporters, both the ABC and the SLC superfamilies and of the drugs they transport. ABC effluxers line the bile canniculi and export drugs into the bile while the SLCs are poised on the basolateral or blood border and influx drugs, both anionic and cationic. Phase 1 and 2 metabolic structures lie in between. Unlike the CYPs, but like the UGTs, both families of transporters handle both endogenous and exogenous compounds. As example, SLCO1B1 (OATP1B1) imports into the hepatic cell both bile salts as well as a variety of drugs including the statins, bosentan, olmesartan and valsartan. This sets up the possibility of many drug interactions when drugs that are SLCO1B1(OATP1B1) inhibitors are added to drugs that are substrates of the same transporter. Gemfibrozil inhibits SLCO1B1 and levels of pravastatin double since the drug can no longer be influxed into the hepatic cell and sent along to the bile by ABCC2. Instead pravastatin stays in the systemic circulation. These transporter-based DDI are just coming into the scientific literature with the development of more specific probes and knock out mice.

ABSTRACT: The role of hepatic transporters in drug elimination. Funk C.

BACKGROUND: Hepatic drug transporters of the solute carrier (OATPs, OAT2, OCT1, NTCP) and ABC transporter superfamilies (MDR1, MRPs, BCRP, BSEP) can significantly modulate drug ADME routes.

OBJECTIVE: The currently available literature was reviewed with focus on hepatic drug transporters, related drug-drug interactions and available tools for transporter assessment and extrapolation to in vivo.

CONCLUSIONS: Hepatic drug transporters have gained additional importance in drug clearance by optimization for basic DMPK properties early on in drug research. However, the lack of selective substrates and inhibitors, proper assessment of the kinetic properties due to interfering passive permeability and multiple binding sites, as well as endogenous transporters present in many cellular assays, are some of the hurdles in studying active drug transport processes. Therefore, data from these in vitro assays have to be carefully evaluated to allow sound extrapolation to the in vivo situation.

COMMENT: The mutant in vivo mouse model or the so-called “knock out” (ko) mouse in which a single gene is deleted or disrupted is increasingly used for the exploration of drug interactions. In the case of the OATP transporters, the so-called SLCO or SLC21 subfamily, the mouse has a single hepatic SLCO transporter, the SLC01B2 tranporter, in place of the 2 human hepatic transporters, SLC01B1 and SLCO1B3 which reside on the basolateral membrane of liver cells (the portal vein side) and influx many common drugs including rifampin and pravastatin.

In this article, details are given of how this “ko” mouse was utilized to test these 2 known SLCO substrates. Conjugated bilirubin was modestly increased, but no jaundice was seen in the ko mice suggesting that other liver transporters take up the slack of importing conjugated bilirubin when SLCO1B2 is lacking. The plasma AUC of rifampin in the KO mouse was 1.7 times that of the wild-type and the plasma clearance of rifampin was reduced. Pravastatin was similarly affected. The authors make it clear that mice SLCOs are different enough from humans that an exact comparison remains unjustifiable, but this in vivo animal model is useful for substrate, inhibitor and inducer analysis as well as drug interaction testing.

ABSTRACT:Mol Pharmacol. 2008 Apr 15 [Epub ahead of print] Targeted Disruption of Murine Organic Anion-Transporting Polypeptide 1b2 (oatp1b2/Slco1b2) Significantly Alters Disposition of Prototypical Drug Substrates Pravastatin and Rifampin. Zaher H, Meyer Zu Schwabedissen HE, Tirona RG, Cox ML, Obert LA, Agrawal N, Palandra J, Stock JL, Kim RB, Ware JA.

Organic anion transporting polypeptide (OATP) 1B1 and 1B3 are widely acknowledged as important and rate-limiting to the hepatic uptake of many drugs in clinical use. Accordingly, to better understand the in vivo relevance of OATP1B transporters, targeted disruption of murine Slco1b2 gene was carried out. Interestingly,Slco1b2(-/-) mice were fertile, developed normally, and did not exhibit any overt phenotypic abnormalities. We confirmed the loss of Oatp1b2 expression in liver using real-time PCR, Western Blot analysis and immunohistochemistry. Oatp1a4 and Oatp2b1, but not Oatp1a1 expression was greater in female Slco1b2(-/-) mice, but expression of other non-oatp transporters did not significantly differ between wildtype and Slco1b2(-/-) males. Total bilirubin level was elevated by 2-fold in the Slco1b2(-/-) mice despite that liver enzymes ALT and AST were normal. Pharmacological characterization was carried out using two prototypical substrates of human OATP1B1 and 1B3, rifampin and pravastatin. After a single intravenous dose of rifampin (1mg/kg), a 1.7-fold increase in plasma AUC was observed while the liver-to-plasma ratio was reduced by 5-fold, and nearly 8- fold when assessed at steady -state conditions after 24 hours of continuous subcutaneous (SC) infusion in Slco1b2(-/-) mice.

Similarly, continuous SC-infusion at low dose rate (8 microg/hr) or high dose rate (32 microg/hr) pravastatin resulted in a 4-fold lower liver-plasma ratio in the in Slco1b2(-/-) mice. This is the first report of altered drug disposition profile in the Slco1b2 knockout mice and suggests the utility of this model for understanding the in vivo role of hepatic OATP transporters in drug disposition.

COMMENT: The fact that desipramine levels were doubled with the addition of bupropion was a surprising clinical event because bupropion’s Ki is so high that it was though not possible for it to significntly inhibit CYP2D6 (IC50 is more than 50uM). In this in vitro study, it is shown that the actual culprits (perpetrators) of CYP2D6 inhibition are 2 of bupropion’s active metabolites (threohydrobupropion and erythrohydrobupropion) that are competitive inhibitors of CYP2D6- with a Ki of 1.7 for the latter in contrast to the Ki of 2.8 for desipramine’s inhibition of CYP2D6. Although this in vitro finding fits nicely into predictive models of the drug interaction, it must be remember that in vitro data sometimes does not translate into in vivo data.

There are documented clinical drug-drug interactions between bupropion and the CYP2D6-metabolised drug, desipramine, resulting in marked (5-fold) increases in desipramine exposure. This finding was unexpected as CYP2D6 does not play a significant role in bupropion clearance, and bupropion and its major active metabolite, hydroxybupropion, are not strong CYP2D6 inhibitors in vitro. The aims of this study were to investigate whether bupropion’s reductive metabolites, threohydrobupropion and erythrohydrobupropion, contribute to the drug interaction with desipramine. In human liver microsomes using the CYP2D6 probe substrate bufuralol, erythrohydrobupropion and threohydrobupropion were more potent inhibitors of CYP2D6 activity (Ki = 1.7 microM and 5.4 microM, respectively) than hydroxybupropion (Ki = 13microM) or bupropion (Ki = 21microM). Further, neither bupropion nor its metabolites were metabolism-dependent CYP2D6 inhibitors. Using the in vitro kinetic constants and estimated liver concentrations of bupropion and its metabolites, modeling was able to predict within two-fold the increase in desipramine exposure observed when co-administered with bupropion. This work indicates that the reductive metabolites of bupropion are potent competitive CYP2D6 inhibitors in vivo and provides a mechanistic explanation for the clinical drug-drug interaction between bupropion and desipramine.

PMID: 18420781

There is a policy comment due out in Science today in which Genelex’s website is prejudicially quoted out of context.   

The quote in the article is:  

“Some Web sites make explicit claims about the utility of CYP450 testing for particular drugs, such as the claim by Genelex that pharmacogenetic testing is “required to effectively prescribe Paxil” (10).  

The actual statement is:  

Paxil (paroxetine) is metabolized through CYP2D6. Pharmacogenetic testing of this pathway serves as an anchor for the intense personalization required to effectively prescribe Paxil and other antidepressant medicines.   http://www.healthanddna.com/professional/paxil.html   

We are narrowing the focus of our response to the focus of the policy comment – SSRIs. However, this focus also points out the continuing problem in healthcare of putting patient information in silos. The fact is just three P450 enzymes – 2D6, 2C9, and 2C19 are responsible for the metabolism of over half of commonly prescribed medications and over half of the population has a variation in one of these key processing enzymes. This information can be used for a lifetime to help caregivers provide more effective and safe treatment regimens. Considering that adverse drug reactions currently kill twice as many as people as vehicle accidents and cost billions of healthcare dollars, a panel that costs less than installing an airbag will certainly prove to be cost-effective when taken in the context of overall medication management not one medication or class at a time. 

Relevant facts and quotes: 

-“The cytochrome P-450 test represents a major advance in the ability to provide the best care possible for depression”  Access April 2, 2008 Mayo Clinic Web site http://www.mayoclinic.org/depression/cytochrome.html  

-Hospitalized psychiatric patients who are poor metabolizers cost $4,000 - $6,000 more in medical care compared to patients with an average metabolizer genotype. Virtually, all antidepressants and antipsychotic medicines are processed by enzymes with a high incidence of poor metabolizers. Journal of Clinical Psycopharmacology 20:246 2000 

- Fifty-nine percent of drugs most commonly cited in ADR studies are processed by enzymes with genes known to have poormetabolizer variants. This is compared to 7% of a random selection of the top selling drugs. (JAMA 286:2270 2001). 

- Multiple studies have found that people with CYP variants require lower doses of the affected medications. Molecular Psychiatry 9:442-473, 2004 - Hundreds of studies stretching back to the 1960s of the genetics of CYP2D6, 2C9, and 2C19 have led the FDA to consider DNA testing of the genes for these enzymes as valid biomarkers of in vivo drug levels. Tox Mech Meth 16:89-99, 2006

 -  “Putting what we know into practice would prevent more disease than worshiping at the altar of randomized trials.”John Concato, MD (Director, VA Clinical Epidemiology Research Center, Yale University)  

Genelex’s Position: 

-  Individuals have a right to learn their genotype and control that information. If patients are denied direct access to this testing they may be reluctant to be tested because they are not confident that the confidentiality of the test results will be adequately protected.

 - Excessive regulation, such as is advocated by the Genetics and Public Policy Center, will impede the already excessively slow rate of adoption of DNA testing for use in medication management. Were this testing adopted at a faster pace there are likely tens of thousands of adverse medical events that would have been prevented.  

- There needs to be symmetry between the level of proof required for the adoption of a technology and the potential risk and cost benefit ratios. 

 -   A peer developed rating system that describes where a given test lies on the continuum of scientific knowledge about the utility, acceptance and proof of that test. In this way individuals would be provided with the tools needed to help them make informed decisions.  

Randomized trials are in progress that will add to our knowledge. In the meantime clinicians and patients who find this information useful should have access to it. Physicians don’t prescribe without knowing the patient’s age, sex, and medical history. In many instances CYP genetics are more important than all these other factors combined. The trend in psychiatry is to utilize polypharmacy and many of these patients have co-morbid conditions for which they are also receiving drugs. The complexity of these medication regimens heightens the need for DNA testing and the use of interpretive software as a step toward comprehensive personalized medication management.

Tadalafil, an oral phosphodiesterase 5 (PDE5) inhibitor, is being investigated as a treatment for pulmonary arterial hypertension. Bosentan is an oral endothelin receptor antagonist widely used in the treatment of pulmonary arterial hypertension.Tadalafil is mainly metabolized by cytochrome P450 (CYP) 3A4, and as bosentan induces CYP2C9 and CYP3A4, a pharmacokinetic interaction is possible between these agents. This open-label, randomized study investigated whether any pharmacokinetic interaction exists between tadalafil and bosentan. Healthy adult men (n = 15; 19-52 years of age) received 10 consecutive days of tadalafil 40 mg once daily, bosentan 125 mg twice daily, and a combination of both in a 3-period, crossover design. Following 10 days of multiple-dose coadministration of bosentan and tadalafil, compared with tadalafil alone, tadalafil geometric mean ratios (90% confidence interval [CI]) for AUCtau and Cmax were 0.59 (0.55, 0.62) and 0.73 (0.68, 0.79), respectively, with no observed change in tmax. Following coadministration of bosentan with tadalafil, bosentan ratios (90% CI) for AUCtau and Cmax were 1.13 (1.02, 1.24) and 1.20 (1.05, 1.36), respectively. Tadalafil alone and combined with bosentan was generally well tolerated. In conclusion, after 10 days of coadministration, bosentan decreased tadalafil exposure by 41.5% with minimal and clinically irrelevant differences (<20%) in bosentan exposure.

PMID: 18305126

COMMENT: This is a short review of an important subject especially for psychiatrists and neurologists. Apart from the usual warnings about inactivating the effects of oral contraceptives (OCs) by the antiepileptic potent CYP3A4 inducers (e.g., phenobarbital, phenytoin, etc), there is also a warning about the induction BY oral contraceptives of the UGTs responsible for lamotrigine and valproate. With traditional week-free OCs, levels of these two drugs will increase during that time ( and conversely they are decreased when OCs are present). There are 3 possible strategies. The authors suggest reducing lamotrigine by 25% during that week free of OCs. Two additional strategies which can also be used for valproate are: 1- switching to an OC that has no week-free period and are continuously given daily (e.g.., Seasonale) and 2- switching to a progesterone only OC which will not induce the UGTs. We know that lamotrigine is a substrate of UGT1A4 and UGT2B7 and that valproate inhibits the latter to cause the substantial increases in lamotrigine thought to predispose Stevens-Johnson syndrome. However, it is not clear which UGT ethinyl estradiol induces to increase the conjugation of lamotrigine. The authors also mention that other antiepileptic undergo glucuronidation, but it is not known if OCs induce the UGT that conjugate them as well. 

ABSTRACT: Seizure2008 Mar;17(2):141-4. Pharmacokinetic interactions between contraceptives and antiepileptic drugs. Sabers A.

The occurrence of bi-directional drug interactions between antiepileptic drugs (AEDs) and combined oral contraceptives (OCs) pose potential risks of un-intended pregnancy and as well as seizure deterioration. It is well established that several of the older AEDs (carbamazepine, phenytoin and phenobarbital), are strong inducers of the hepatic cytochrome P450 (CYP) 3A4 enzyme system, and are associated with increased the risk of contraceptive failure. In addition, it is demonstrated that also some of the newer AEDs, oxcarbazepine and topiramate influence on the pharmacokinetics of OCs, which is thought to be due to a more selective induction of subgroups of the hepatic enzyme system. Estrogens containing OCs induce the glucuronosyltransferase and may reduce the plasma levels and the effect of AEDs cleared by glucuronidation. This has been most intensively studied for lamotrigine but also other AEDs, which undergoes glucuronidation processes, such as valproate and oxcarbazepine, may be affected by OCs. The magnitude of the drug-drug interactions show in general wide inter-individual variability and the change in the elimination rate is often unpredictable and can be influenced by a number of co-variants such as co-medication of other drugs, as well as genetic and environmental factors. It is therefore recommended that change in OC use is assisted by AED monitoring whenever possible. 

PMID: 18206393

ABSTRACT: