as used herein the following terms shall have the definitions set forth below . as used herein , the term “ cell ” includes both primary cells as well as established cell lines ( e . g ., human embryonic kidney hek293 cells , chinese hamster ovary cho , madin - darby canine kidney cells mdck , pig kidney epithelial cells llc - pk1 , human epithelial colorectal adenocarcinoma cells caco - 2 and chinese hamster lung fibroblast v79 cells ). as used herein , the term “ drug transporter protein ” refers to a membrane bound transport protein that includes , but is not limited to atp binding cassette ( abc ) transporters and solute carrier ( slc ) transporters . as used herein , the term “ drug metabolizing enzyme ” includes , but is not limited to , cytochromes such as cytochrome ( cyp ) p450 ; udp - glucouronyl transferase and other non - cyp drug metabolizing enzymes such as alcohol dehydrogenase , monoamine oxidase and aldehyde oxidase . as used herein , the term “ detectable ” means that the activity of a selected probe substrate in cells transfected with a drug transporter protein and / or drug metabolizing enzyme shall be higher than the activity of the same probe substrate in cells transfected with empty vector ; desirably , the difference in activity will be at least 5 - fold . as used herein , the use of upper case letters in transporter nomenclature identifies the human protein / gene , i . e ., mrp2 / abcc2 , etc . ; smaller case letters indicate the transporter derives from a preclinical ( nonhuman mammalian ) species , i . e ., mrp2 / abcc2 , etc . unless otherwise specified , a gene is derived from any species ( e . g ., human or other mammal ). as used herein , the terms “ oatp1b1 ” “ oatp2 ” and “ slco1b1 ” are interchangeable and refer to a human protein / gene that corresponds to the nonhuman protein / gene oatp2 . unless noted otherwise , reference to oatp1b1 is to oatp1b1 * 1b . as used herein , the terms “ oat1 ” and “ slc22a6 ” are interchangeable and refer to an organic anion transporter 1 . unless noted otherwise , reference to oat1 is to the full length cdna encoding with 563 amino acids ( also referred to herein as “ oat1 long ”). exemplary abc transporters include , but are not limited to those listed below in table 1 . cells suitable for use in the present invention include mammalian cells , for example , as derived from human or non - human ( e . g ., mouse , rat , dog , monkey , hamster and pig , etc .). in certain embodiments , the cells are hepatocytes , or endothelial cells . gene delivery systems for introducing gene ( s ) into a population of cells are known to a skilled artisan . virus - based gene delivery methods may be used but require special handling of the cells due to safety concerns . although lipid - based transfection methods may be used , lipid - based transfection reagents are relatively costly and such methods are not amenable to large - scale manufacturing processes . additionally , lipid - based transfection methods result in relatively low gene delivery efficiency and relatively delayed protein expression ( generally 72 to 96 hours post transfection ) ( data not shown ). electroporation ( ep ) is preferable as it is amenable to large - scale manufacturing processes and avoids the safety issues of viral - based gene delivery methods . further , ep results in relatively efficient gene delivery . after gene delivery into a population of cells , gene ( s ) encoding a drug transporter protein and / or a drug metabolizing enzyme will be overexpressed such that activity of the protein ( s ) encoded therefrom are detectable following thaw from cryopreservation . drug candidates can be tested to determine if any are substrates or inhibitors of the protein ( s ) encoded from the overexpressed gene ( s ) by incubation of the recombinant cells therewith . in particular , if a drug candidate is a substrate of a drug transporter protein and / or a drug metabolizing enzyme , the drug candidate will be affected . for instance , if the drug candidate is a substrate of a drug transporter protein , the drug candidate will be translocated in or out of the recombinant cell via the drug transporter protein . however , if the drug candidate is an inhibitor of the drug transporter protein , the drug candidate will inhibit translocation of a substrate of the drug transporter protein in or out of the recombinant cell . alternatively , assays can be conducted using whole cells or subcellular fractions thereof ( microsome / cytosol ). additionally , recombinant cells of the present invention may be further transfected with rnai or sirna of the transiently overexpressed gene ( s ) to knockdown / knockout the expression thereof as is desirable for certain assays . primary cells ( e . g ., hepatocytes ) can be transfected with rnai or sirna directed against any abc transporters , slc transporters or any other drug metabolizing enzymes to knockdown / knockout the expression of specific genes . in one aspect ( 1 ), the disclosure provides a cryopreserved recombinant cell including one or more transiently overexpressed genes encoding a protein selected from the group consisting of a drug transporter protein and a drug metabolizing enzyme , or a combination thereof wherein activity of the drug transporter protein or the drug metabolizing enzyme or combinations is detectable in a population of the cryopreserved recombinant cell following thaw from cryopreservation . in an aspect ( 2 ), the disclosure provides the invention of aspect ( 1 ), wherein said one or more genes encodes a drug metabolizing enzyme . in an aspect ( 3 ), the disclosure provides the invention of aspect ( 2 ), wherein the drug metabolizing enzyme is selected from the group consisting of cytochrome p450 , udp - glucouronyl transferase , alcohol dehydrogenase , monoamine oxidase and aldehyde oxidase . in an aspect ( 4 ), the disclosure provides the invention of aspect ( 1 ), which transiently overexpresses one or more genes encoding a protein selected from the group consisting of an atp binding cassette transporter and a solute carrier transporter protein . in an aspect ( 5 ), the disclosure provides the invention of aspect ( 4 ), wherein said one or more genes is selected from the group consisting of mdr1 / mdr1a / mdr1b , mrp1 / mrp1 , mrp2 / mrp2 , mrp3 / mrp3 , mrp4 / mrp4 , mrp5 / mrp5 , mrp6 / mrp6 , mrp7 / mrp7 , mrp 8 / mrp8 , bcrp / bcrp , bsep / bsep , oatp2 / oatp2 , oatp1b3 / oatp1b3 , oat1 / oat1 , oat2 / oat2 , oat3 / oat3 , oat4 / oat4 , oct1 / oct1 , oct2 / oct2 , oatp1 / oatp1 , pept1 / pept1 , pept2 / pept2 , octn1 / octn1 , octn2 / octn2 , mate1 / mate1 , mate2k / mate2 , urat1 / urat1 , asbt / asbt , and ntcp / ntcp . in an aspect ( 6 ), the disclosure provides the invention of aspect ( 4 ), wherein said one or more genes encodes a protein that is an atp binding cassette transporter selected from the group consisting of mdr1 / mdr1a / mdr1b , mrp1 / mrp1 , mrp2 / mrp2 , mrp3 / mrp3 , mrp4 / mrp4 , mrp5 / mrp5 , mrp6 / mrp6 , mrp7 / mrp7 , mrp8 / mrp8 , bcrp / bcrp , and bsep / bsep . in an aspect ( 7 ), the disclosure provides the invention of aspect ( 4 ), wherein said one or more genes encodes a protein that is a solute carrier transporter selected from the group consisting of oatp2 / oatp2 , oatp1b3 / oatp1b3 , oat1 / oat1 , oat2 / oat2 , oat3 / oat3 , oat4 / oat4 , oct1 / oct1 , oct2 / oct2 , oct3 / oct3 , oatp1 / oatp1 , pept1 / pept1 , pept2 / pept2 , octn1 / octn1 , octn2 / octn2 , mate1 / mate1 , mate2k / mate2 , urat1 / urat1 , asbt / asbt , and ntcp / ntcp . in an aspect ( 8 ), the disclosure provides the invention of aspect ( 4 ), wherein said one or more genes is selected from oatp1b1 * 1a , oatp1b1 * 1b , oatp1b3 , oat1 , oat3 , oct1 , oct2 , mate1 and mate2k . in an aspect ( 9 ), the disclosure provides the invention of aspect ( 1 ), wherein the one or more genes is derived individually from human or an animal species selected from mouse , rat , guinea pig , dog , and monkey . in an aspect ( 10 ), the disclosure provides the invention of aspect ( 1 ), wherein said cell is derived from a mammal . in an aspect ( 11 ), the disclosure provides the invention of aspect ( 10 ), wherein said cell is selected from the group consisting of hek293 , cho , mdck , llc - pk1 , caco - 2 and v79 cells . in an aspect ( 12 ), the disclosure provides the invention of aspect ( 10 ), wherein the mammal is selected from the group consisting of human , monkey , dog , rat , mouse , porcine and hamster . in an aspect ( 13 ), the disclosure provides the invention of aspect ( 1 ), wherein said cell comprises a hepatocyte . in an aspect ( 14 ), the disclosure provides the invention of aspect ( 1 ), wherein said cell comprises an endothelial cell . in an aspect ( 15 ), the disclosure provides the invention of aspect ( 1 ), wherein activity of the protein ( s ) is detectable in a population of said cell at least 24 hours post plating following thaw from cryopreservation . in an aspect ( 16 ), the disclosure provides the invention of aspect ( 1 ), wherein activity of the protein ( s ) is detectable in a population of said cell at least 48 hours post plating following thaw from cryopreservation . in an aspect ( 17 ), the disclosure provides the invention of aspect ( 1 ), wherein activity of the protein ( s ) is detectable in a population of said cell at least 72 hours post plating following thaw from cryopreservation . in another aspect ( 18 ), the disclosure provides a process of preparing transiently transfected recombinant cells which transiently overexpresses one or more genes encoding a protein selected from a drug transporter protein and a drug metabolizing enzyme including transiently transfecting cells with one or more genes encoding a drug transporter protein or a drug metabolizing enzyme and cryopreserving the transiently transfected recombinant cells within 48 hrs of transfection . in an aspect ( 19 ), the disclosure provides the invention of aspect ( 18 ), wherein the transient transfection step includes electroporation . cells were cultured under standard sterile practices for cell culture , and transiently transfected using ep . following ep , cells were assayed for protein activity both before as well as after being frozen , thawed and plated . as detailed below , cells cultured in suspension and adherent cell cultures were both successfully transiently transfected and exhibited substantial activity of the recombinant protein following thaw from cryopreservation . in brief , on day 1 , freestyle 293 cells and 293 - f cells were each passaged into appropriate sized shaker flasks at a density of 0 . 7 - 1 . 0 × 10 6 cell / ml using supplemented cd293 medium ( i . e ., cd293 medium ( available from gibco , cat . no . 11913 - 019 , life technologies corp ., carlsbad , calif .) supplemented with 4 mm l - glutamine ( available from gibco , cat . no . 25030 - 081 , life technologies corp ., carlsbad , calif .)) or supplemented excell ™ 293 serum free media ( available from sigma , cat . no . 14571c , sigma - aldrich , st . louis , mo .) supplemented with 6 mm l - glutamine cell viability and cell number were determined using a cellometer ( available from nexcelom bioscience , lawrence , mass .). on day 2 , ep of cells was executed . in short , following a determination of cell viability and cell density , cells were pelleted down by spinning at 100 g for 5 min , after which the media was aspirated and cells resuspended in 30 ml ep buffer ( available from maxcyte , cat . no . b201 , maxcyte inc ., gaithersburg , md .). the cell suspension was transferred to 50 ml falcon tubes , pelleted down as described above , and resuspended in an appropriate amount of ep buffer to reach 100 × 10 6 cells / ml which was used as the cell stock . dnas to be used for ep were prepared in sterile water at a final concentration of 5 mg / ml . for each sample , 0 . 4 ml of cell stock and dna was placed in a sterile 1 . 5 ml eppendorf tube resulting in a final concentration of 200 μg / ml ( table 4 ) or 300 μg / ml dna ( table 10 and table 11 ) and cell density of 40 × 10 6 cells per sample . samples were transferred into an oc - 400 processing assembly ( available from maxcyte , cat . no . oc - 400r , maxcyte inc ., gaithersburg , md .) which followed the manufacture instructions for ep of hek cells . following ep , the cells were carefully pipetted out and transferred into the bottom of a 125 ml shaker flask and incubated for 20 min at 37 ° c . with 8 % co 2 , after which pre - warmed 40 ml cd293 media was added into the shaker flask to reach cell density at 1 × 10 6 cells / ml . the cells were incubated for 30 min at 37 ° c . and 8 % co 2 . after 30 min recovery , cell viability and cell density were determined . a portion of cells ( i . e ., 20 × 10 6 cells ) was used for plating and the rest was cryopreserved , or all of the cells were cryopreserved . it is contemplated that recombinant cells may be cryopreserved within 48 hrs of transfection and exhibit activity of protein ( s ) encoded from transfected gene ( s ) at a detectable level following thaw from cryopreservation . for plating cells following ep , 20 × 10 6 cells were pelleted down by spinning at 100 g for 5 min and then resuspended in 20 ml pre - warmed plating media ( dmem with high glucose ( available from gibco , cat . no . 11965092 , life technologies corp ., carlsbad , calif . ), supplemented with 0 . 1 mm non - essential amino acids ( available from gibco , cat . no . 11140050 , life technologies corp ., carlsbad , calif . ), 10 % fbs ( available from safc biosciences , cat . no . 12016c , sigma , st . louis , mo .)) ( cell density of 1 × 10 6 cells / me . cells were placed in 24 - well tissue culture plates poly - d - lysine coated , corning biocoat ™ ( available from corning life sciences , tewksbury , mass .) at a density of 0 . 2 × 10 6 cells / well and 0 . 4 × 10 6 cells / well and incubated at 37 ° c . with 8 % co 2 so as to determine the impact of seeding density on uptake activity . media was replaced 4 hours later and then every 24 hours until the day of assaying . on days 4 , cells were assayed for oatp1b1 activity as described below . for cryopreservation , cells were pelleted then resuspended in freshly prepared ice - cold freezing media ( 9 parts supplemented cd293 medium and 1 part dmso which was syringe filtered to sterilize ) at a density of 10 × 10 6 cell / ml . cryo vials were filled with 1 ml of this cell suspension , and placed on ice - cold mr frosty freezing container ( available from thermal scientific ), which was stored in − 80 ° c . freezer overnight after which the vials were transferred into liquid nitrogen . cryopreserved cells were assayed for oatp1b1 activity as described below . in brief , on day 1 , cryopreserved cells were removed from liquid nitrogen to dry ice , and then thawed in a water bath at 37 ° c . for about 2 min . cells were transferred into 10 ml of plating media as described above which is pre - warmed to a temperature of about 37 ° c . and the viability and cell density determined . cells were pelleted down and resuspended in supplemented dmem media at a cell density of 1 × 10 6 viable cells / ml . cells were plated in the same manner described above for plating cells following ep ( which had not been cryopreserved ) and assayed for oatp1b1 activity at 24 , 48 and 72 hrs following plating thereof . in brief , hek293 cells were cultured in 5 layer corning ® cellstack ® ( available from corning inc . life sciences , tewksbury , mass .) using plating media containing dmem ( high glucose ) available from gibco cat . no . 11965118 , life technologies corp ., carlsbad , calif . ; penicillin - streptomycin ( 10 , 000 units / ml ) available from gibco cat . no . 15140 - 122 , life technologies corp ., carlsbad , calif . ; l - glutamine ( 200 mm ) available from gibco cat . no . 25030 - 081 , life technologies corp ., carlsbad , calif . ; sodium pyruvate , available from gibco cat . no . 11360 , life technologies corp ., carlsbad , calif . ; fbs available from sigma - aldrich corp ., st . louis , mo . in a ratio of 100 : 1 : 1 : 1 : 10 . on day 1 , about 24 hrs before ep , hek293 cells were trypsinized , cell viability and cell number determined after which cells were passaged to fresh multilayer chamber flasks at 30 - 40 % confluency . cells were incubated at 37 ° c . with 5 % co 2 . on day 2 , ep of cells was executed . in short , cells were harvested , cell viability and cell number determined after which cells were pelleted down by spinning at 100 g for 5 min and the media aspirated . cells were resuspended in ep buffer and pelleted down by spinning at 100 g for 5 min , then resuspended in an appropriate amount of ep buffer to reach 50 × 10 6 cells / ml which was used as the cell stock . dnas to be used for ep were prepared in sterile water at a final concentration of 5 mg / ml . for each sample used for oc - 400 processing assembly , 0 . 4 ml of cell stock and dna was placed in a sterile 1 . 5 ml eppendorf tube resulting in a final concentration of 50 μg / ml , 100 μg / ml , 200 μg / ml or 400 μg / ml dna as indicated in fig5 - 9 and cell density of 40 × 10 6 cells per sample . for each sample used for cl - 2 processing assembly , 10 ml of cell stock and dna was placed in 50 ml sterile conical tube resulting in a final concentration of 100 μg / ml dna . samples were transferred into an oc - 400 or cl - 2 processing assembly ( available from maxcyte , cat . no . oc - 400r and cl2 - r , maxcyte inc ., gaithersburg , md .) which followed the manufacture instructions for ep of hek cells . following ep , the cells were carefully pipetted out and transferred into 6 - well tissue culture plates and incubated for 20 min at 37 ° c . with 5 % co 2 , after which cells were removed and placed in a 50 ml conical tube containing pre - warmed plating media . cell viability and cell density were determined . a portion of cells ( i . e ., 20 × 10 6 cells ) was used for plating and the rest was cryopreserved . for plating cells following ep , cells were pelleted down by spinning at 100 g for 5 min and then resuspended in pre - warmed plating media ( cell density of 1 × 10 6 cells / ml ). cells were placed in 24 - well tissue culture plates ( poly - d - lysine coated , corning biocoat ™ ( available from corning life sciences , tewksbury , mass .)) at a density of 0 . 4 × 10 6 cells / well and incubated at 37 ° c . with 5 % co 2 . media was replaced 4 hours later and then every 24 hours until the day of assaying . on days 4 and 6 , cells were assayed for oatp1b1 activity . for cryopreservation , cells were pelleted then resuspended in freshly prepared ice - cold freezing media ( 9 parts plating medium and 1 part dmso which was syringe filtered to sterilize ) at a density of 10 × 10 6 cell / ml . cryo vials were filled with 1 ml of this cell suspension , and placed on ice - cold mr frosty freezing container ( available from thermal scientific ) stored in − 80 ° c . freezer overnight after which the vials were stored in liquid nitrogen . cryopreserved cells were assayed for oatp1b1 activity . notably , cells were plated in the same manner described above for plating cells following ep ( which had not been cryopreserved ) and assayed for oatp1b1 activity ( as described below ) at 48 hrs following plating thereof . in brief , substrate solution was prepared for oatp1b1 * 1a and oatp1b1 * 1b using 2 μm estradiol - 17β - glucuronide ( 99 % of cold e17βg and 1 % of [ 3 h ]- e17βg ); for oatp1b3 using 2 μm cck - 8 ( 99 % of cold cck - 8 and 1 % of [ 3 h ]- cck - 8 ); for oat1 short using 1 μm para - aminohippurate ( pah ) ( 90 % of cold pah and 10 % of [ 3 h ]- pah ); for oat1 long using 1 μm or 3 μm para - aminohippurate ( pah ) ( 90 % of cold pah and 10 % of [ 3 h ]- pah ); for oat3 using 1 μm or 2 μm estrone - 3 - sulfate ( 99 % of cold e3s and 1 % of [ 3 h ]- e3s ); for oct1 and oct2 using 30 μm tetraethylammonium bromide ( 100 % [ 14 c ]- tea ); for mate1 and mate2k using 10 μm metformin ( 100 % [ 14 c ]- metformin ) or 10 μm tetraethylammonium bromide ( 100 % [ 14 c ]- tea ); in krebs - henseleit buffer ph 7 . 4 ( available from sigma , cat . no . k3753 , sigma - aldrich , st . louis , mo .) and incubated at 37 ° c . for at least 20 min . culture media was aspirated from cells to be assayed and cells washed thrice with pre - warmed khb buffer . cells were subsequently incubated with uptake buffer at 37 ° c . for 10 min . for mate1 and mate2k , cells were washed and pre - incubated with khb buffer containing 20 mm nh 4 cl for 10 min . assays were initiated by adding 0 . 3 ml substrate solution into each well and incubated at 37 ° c . for 5 min with samples for oct1 and oct2 incubated for 10 min . the reaction was quickly stopped after the incubation period by aspirating substrate solution from cells then washing cells thrice with cold uptake buffer . cells were then incubated with lysing solution ( 0 . 1 % sds with 0 . 1 % v / v 1m naoh in dulbecco &# 39 ; s phosphate - buffered saline ( dpbs ) buffer ) for 15 - 20 minutes while being shaken . the substrate solution was triturated and 0 . 4 ml of the resultant cell lysis placed in 5 ml scintillation tube with 5 ml of scintillation liquid for analysis with scintillation counter . as illustrated in fig1 , cell viability dropped 1 - 5 % after ep relative to that of the cell stock . additionally , after cryopreservation , cell viability dropped an additional 10 - 15 % relative to that after ep . nonetheless , even after ep and thaw from cryopreservation , cell viability is greater than 75 %. cell morphology and uptake activity was examined following cryopreservation after 30 min recovery and 24 hrs recovery post transfection . table 5 illustrated cell morphology and uptake activity with 24 hrs recovery was reduced compared to 30 min recovery . cell morphology and confluency of transfected cells thawed from cryopreservation were examined after various periods of time following plating at a density of 0 . 4 × 10 6 cells per well in 24 - well poly - d - lysine coated corning biocoat ™ plates . in particular , fig2 illustrates oatp1b1 transiently transfected cells cultured at 4 hrs , 24 hrs and 72 hrs post plating . additionally , cell confluency at 24 hrs , 48 hrs and 72 hrs post - plating of these cells is recorded in table 6 below . as illustrated in fig5 , the expression of green fluorescent protein ( gfp ) in adhesion hek293 cells was increased with increasing concentration of dna . additionally , gfp expression increased at the 48 hr timepoint relative to the 24 hr timepoint . in particular , gfp transfection efficiency by ep achieved 100 % at 24 hrs with 200 μg / ml dna and 100 % fluorescent cell staining at 48 hrs with 100 μg / ml dna . hence , gfp protein expression level in transfected cells increased with increased dna concentration and at 48 hrs relative to 24 hrs . uptake activity of suspension cultured 293 cells transfected with oatp1b1 ( poatp1b1 ) and control vector ( pcmv ) were assayed at various time points following ep . in brief , transfected cells were plated at a density of 0 . 4 × 10 6 cells / well in 24 - well poly - d - lysine coated corning biocoat ™ plates following ep or after thaw from cryopreservation . oatp1b1 uptake activity and uptake ratio were determined using probe substrate , estradiol - 17β - glucuronide , in both fresh plated cells (“ fresh ”) and cryopreserved cells (“ cryo ”) at various timepoints post plating as detailed in table 7 below . oatp1b1 uptake activity and uptake ratio in transfected cells following thaw from cryopreservation is consistent with those in freshly plated transfected cells . in both cells types 293 - f and fs293 , the highest uptake activity and uptake ratio is observed at 24 hrs post plating . morphology and cell confluency of transfected cells ( fs293 or 293 - f ) were examined at 24 hrs , 48 hrs and 72 hrs post - plating in 24 - well poly - d - lysine coated corning biocoat ™ plates at plating density of either 0 . 4 × 10 6 cells / well or 0 . 2 × 10 6 cells / well after thaw from cryopreservation . cell confluency at 24 hrs post - plating are summarized below in table 8 . cell confluency at 48 hrs and 72 hrs are similar to those achieved at 24 hrs ( data not shown ). additionally , fig3 provides images of transfected cells plated at ( a ) 0 . 4 × 10 6 cells per well and ( b ) 0 . 2 × 10 6 cells per well 24 hrs post - plating following thaw from cryopreservation at a confluence of 90 - 95 % and 60 - 70 %, respectively . for optimal assay performance , plating cells at a density of 0 . 4 × 10 6 is preferable to that of 0 . 2 × 10 6 as it achieves higher cell confluency and higher uptake activity . as illustrated in fig6 , when using adhesion hek293 cells , cell viability post ep dropped with increasing amounts of dna used in ep . nonetheless , cell viability following transfection with poatp1b1 was ranged from 89 % to 77 % and that following transfection with empty vector was 90 %. as illustrated in fig7 , when using adhesion hek293 cells , oatp1b1 mediated uptake of estradiol - 17β - glucuronide in the fresh plated transient transfected adhesion hek293 cells is time - dependent . notably , uptake activity and uptake ratio increased with increasing amounts of dna used in ep . however , oatp1b1 mediated uptake of estradiol - 17β - glucuronide reduced at the 96 hr timepoint relative to the 48 hr timepoint . further , as illustrated in fig8 , the signal to noise ratio ( i . e ., uptake ratio ) of estradiol - 17β - glucuronide increased with the increase of amount of dna and assay incubation time , in adhesion hek293 cells transfected with oatp1b1 relative to empty vector at 48 hrs post ep . as illustrated in fig9 , when using adhesion hek293 cells , estradiol - 17β - glucuronide uptake in oatp1b1 transiently expressed hek293 cells using small scale ep device and large scale ep device is consistent for both uptake activity and signal to noise ratio ( i . e ., uptake ratio ). 100 μg / ml dna was used in the experiments . as illustrated in fig1 , when using adhesion hek293 cells , oatp1b1 uptake activity is compared between the cells transfected using traditional lipid transfection reagent ( control : lipofectamine 2000 , available from invitrogen ) and ep using stx , maxcyte inc ., gaithersburg , md . notably , cells transfected using ep resulted in a pronouncedly greater signal to noise ratio relative to those cells transfected with lipid transfection reagent . as illustrated in fig1 , when using adhesion hek293 cells , oatp1b1 uptake activity in both freshly plated ep transfected cells and cells following thaw from cryopreservation was detectable . uptake activity of suspension cultured 293 cells transfected with oatp1b1 * 1a , oatp1b1 * 1b , oatp1b3 , oat1 long , oat1 short , oat3 , oct1 , oct2 , mate1 , mate2k or control vector ( pcmv ) were assayed at 24 hrs post plating after thaw from cryopreservation . in brief , the transfected cells were plated at a density of 0 . 4 × 10 6 cells / well in 24 - well poly - d - lysine coated corning biocoat ™ plates following ep and after thaw from cryopreservation . slc transporter uptake activity and uptake ratio were determined using probe substrates as indicated at 24 hrs post plating as detailed in table 10 below . as reflected in table 10 above , the recombinant cells exhibited strong uptake activity towards their specific prototypical substrate each of which had an uptake ratio above 10 . notably , an uptake ratio above 5 indicates a successful process . as reflected in table 11 , the post - thaw viability for recombinant cryopreserved cells was determined to be above 90 %. each of these recombinant cells as well as a control vector ( pcmv ) was examined 24 hrs post - plating ( after cryopreservation ). confluency for each of these cells 24 hrs post - plating was 85 % or greater as reflected in table 12 below . as illustrated in fig1 - 19 and tables 13 - 14 , the kinetic and inhibition profiles examined in cryopreserved recombinant cells expressing a transporter protein was consistent with reported values . specifically , as illustrated in fig1 a - 13c , the kinetics of pah uptake by recombinant cells expressing oat1 and inhibition profile of probenecid thereof is consistent with reported values . as illustrated in fig1 a - 14c , the kinetics of e3s uptake by recombinant cells expressing oat3 and inhibition profile of probenecid thereof is consistent with reported values . as illustrated in fig1 a - 15f , the kinetics of tea and metformin uptake by recombinant cells expressing oct1 as well as inhibition profile thereof is consistent with reported values . as illustrated in fig1 a - 16e , the kinetics of tea and metformin uptake by recombinant cells expressing oct2 as well as inhibition profile is consistent with reported values . as illustrated in fig1 a - 17f , the kinetics of e17βg , e3s and rosuvastatin uptake by recombinant cells expressing oatp1b1 * 1a as well as inhibition profile of e17βg uptake by cyclosporin a is consistent with reported values . as illustrated in fig1 a - 18e , the kinetics of e17βg , e3s and rosuvastatin uptake by recombinant cells expressing oatp1b1 * 1b as well as inhibition profile of e17βg uptake by cyclosporin a is consistent with reported values . as illustrated in fig1 a - 19e and tables 13 - 14 , the kinetics of cck - 8 , e17βg and rosuvastatin uptake by recombinant cells expressing oatp1b3 as well as inhibition profile of cck - 8 uptake by cyclosporin a is consistent with reported values . it will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof . it is understood , therefore , that this invention is not limited to the particular embodiments disclosed , but is intended to cover modifications that are within the spirit and scope of the invention , as defined by the appended claims .
the present invention discloses cryopreserved recombinant cells for screening drug candidates that transiently overexpress one or more drug transporter proteins and / or drug metabolizing enzymes . advantageously , such cells provide a cost - efficient consumable product that streamlines the process of screening whether drug candidates are substrates or inhibitors of drug transporter proteins and / or drug metabolizing enzymes .