This is a prospective, non-randomized, phase I study design evaluating the in vivo activities and expression of OCT1 and BCRP in mammary gland of lactating women at three time points postpartum.

Each woman will receive a single oral dose of cimetidine 200 mg on each of 3 study days (3-5 weeks, 3-4 months, and 6-8 months postpartum) followed by serial collection of blood, urine and breast milk samples over 12-hours. Cimetidine concentrations will be assay using a validated LC/MS/MS assay. Subjects will be genotyped for OCT1 and BCRP. Mammary epithelial cells will be isolated from breast milk and transporter expression will be quantified. Each woman will serve as her own control.

Inclusion criteria

1. Healthy postpartum women
2. 18-50 years of age and their infants
3. Able to provide written informed consent

Exclusion criteria

1. Receiving cimetidine within the 3 days prior to each study day. Concomitant administration of cimetidine will confound interpretation of study results.
2. Hypersensitivity to cimetidine Patients with known allergic reactions to cimetidine will be excluded for safety reasons
3. Receiving medication known to interact with cimetidine: OCT, BCRP, CYP3A4, CYP2D6, CYP1A2 and CYP2C9 substrates (e.g. amiodarone, clopidogrel, diazepam, ketoconazole, metformin, nifedipine, phenytoin, procainamide, theophylline,tricyclic antidepressants and warfarin) Patients with drug interactions will be excluded for safety reasons.
4. Receiving BCRP inhibitors/inducers (afatinib, aripipraxole, axitinib, cimetidine, cyclosporine, curcumin/tumeric, delavirdine, efavirenz, elacridar, elvitegravir, etravirine, FTC, 5-fluorouracil, fluvastatin, imatinib, lanzoprazole, lapatinib, lopinavir, maraviroc, nelfinavir, nebicapone, nilotinib, novobiocin, oltipraz, omeprazole, pantoprazole, phenobarbital, promazine, rabeprazole, riboflavin, rifampicin, risperidone, saquinavir, sirolimus, sorafenib, sulfasalazine, sunitinib, tacrolimus, tariquidar, telaprevir, telatinib, teriflunomide, tolcapone, triflunomide, trametinib, trifluoperazine, venlafaxine, zidonuvir), OCT1 inhibitors/inducers (acyclovir, amantadine, amiloride, amitriptyline, bucindolol, carvedilol, chlorpheniramine, chlorpromazine, cimetidine, citalopram, clonidine, clopidogrel, clotrimazole, clozapine, cocaine, corticosterone, cyclosporine, daclatasvir, darunavir, desipramine, dextromethorphan, diltiazem, disopyramide, dronedarone, efavirenz, famotidine, fentanyl, fluvoxamine, formoterol, fuloxetine, griseofulvin, doxazosine, ganciclovir, guanfacine, imipramine, indinavir, isavuconazole, itraconazole, ketoconazole, lamotrigine, lasmiditan, levofloxacin, levomepromazine, lidocaine, maprotiline, methylnicotinamide, morphine, moxifloxacin, nefazodone, nelfinavir, nevirapine, nicotine, nomifensine, ondansetron, oxybutynin, paroxetine, pentamidine, phenoxybenzamine, prazosin, probenecid, procainamide, propafenone, pyrazinamide, quetiapine,quinidine, quinine, reboxetine, remoxidpride, reseripine, rifampicin, ritonavir, salmeterol, saquinavir, tramadol, trimethoprim, trimipramine, verapamil) Inhibitors and inducers of the drug transporters will confound data analysis and interpretation.
5. Kidney disease could confound data analysis and interpretation. Therefore, patients with known kidney disease with documented renal function impairment will be excluded from the study. Current serum creatinine > 1.2 mg/dL in their medical record will be excluded.
6. Known liver disease Liver disease will confound data analysis and interpretation. Therefore, patients with known significant liver disease will be excluded from the study. Current ALT exceeding 2-times the upper limit of normal in their medical record will be excluded.
7. Inability to fast for 4 hours prior to the study. To limit PK variability across study days, subjects will be requested to fast for 4 hours prior to each study day.
8. Smokers (tobacco or other nicotine containing products Nicotine interacts with OCT1 and will confound data analysis and interpretation

We are committed to share data with the academic community and researcher-at-large. We will adhere to the NIH Grant Policy on Sharing Research Data and Resources. That is, after final data collection and analysis and after full peer review is performed on our data, the de-identified data generated by this project will be made available when requested by the academic community and researchers-at-large contingent upon Material Transfer Agreements with the University of Washington.

After peer review and publication of data.

We are committed to share data with the academic community and researcher-at-large. We will adhere to the NIH Grant Policy on Sharing Research Data and Resources. That is, after final data collection and analysis and after full peer review is performed on our data, the de-identified data generated by this project will be made available when requested by the academic community and researchers-at-large contingent upon Material Transfer Agreements with the University of Washington.