Patent Abstract:
there is provided a microparticle composition suitable for molecular imaging , the composition comprising microparticles , wherein the microparticles comprise : a core microparticle structure having a central area and a shell , and wherein the core microparticle structure comprises a phosphatidylcholine lipid : a phosphatidylethanolamine lipid comprising at least one maleimide moiety ; and an alkoxylated fatty acid .

Detailed Description:
the invention will now be described in more detail by way of example only and with reference to the following figures . fig1 . frozen section immunohistochemistry of the mouse heart . representative example from mice 6 h post lipopolysaccharide ( lps ) pre - treatment . magnification 200 ×. low power magnification ( low mag ) 40 ×. fig2 . e - selectin ( esel ) targeting microparticles & amp ; in vitro validation . ( a ) a schematic representation of esel targeting particles . ( b ) morphology of esel targeting particles . ( c ) size distribution of esel targeting particles . each bar represents mean ± sem , n = 5 batches of particles prepared on separate occasions . ( d ) in vitro validation of esel targeting particles . each bar represents mean ± sem of attached particle density relative to that of esel targeting particles on dish e . fig3 . in vivo validation of the esel targeting microparticles . intravital microscopy ( ivm ) of esel targeting particles in the mouse cremaster . ( a ) elimination of circulating - particles against time in the wild - type ( wt ) and esel knock - out ( ko ) group of mice . data point represents mean ± sd . ( b ) accumulation of attached particles against time in 5 wt and 5 esel ko mice . ( c ) silicon intensifier target ( sit ) camera images ( fluorescence microscopy , magnification 200 ×) of attached particles in a wt ( i ) and esel ko ( ii ) mice . ( d ) quantification of attached particles on the cremaster venular wall . each point represents one animal ; bars represent group mean ± sem . ( e ) confocal microscopy of esel targeting particles in the mouse cremaster venule . esel in green ( i , ii ), particles in red ( iii , iv ), endothelium ( pecam - 1 ) in grey ( v , vi ). combined all 3 components ( vii , viii ). fig4 . real - time ultrasound ( us ) molecular imaging of esel expression in the mouse heart . ( a ) sequential 14 mhz contrast pulse sequencing ( cps ) images of the heart in end - diastole parasternal short - axis ( psa ) view , in a ( 1 ) wt and ( 2 ) esel ko mouse pre - treated with lps , respectively . tics of the lv cavity ( from region of interest ( roi ) c ) and myocardium ( from roi m ) for the respective animal are shown beneath ; each data point represents background subtracted mean signal intensity ( i ) ± sd ; suggested bolus ( b ), distribution ( d ) and elimination - phase ( e ) of the time signal intensity curve ( tic ) are indicated . ( b ) psa , parasternal long - axis ( pla ) and apical 4 - chamber ( a4c ) views of the heart at 14 and 7 mhz cps , & gt ; 20 min post particle administration ( when freely circulating - particles have cleared from the blood pool ( left ventricular ( lv ) cavity )). animal , gain and mi were the same between both frequencies . arrow indicates mid anteroseptal wall . baseline images before particle administration are shown in fig6 . fig5 a . real - time us molecular imaging of esel expression in the mouse abdomen . combined 14 mhz ‘ cps - contrast only ’ and ‘ b - mode images ’ before ( i , ii , v , vi ) and ≈ 30 min post ( iii , iv , vii , viii ) particle administration . kidney ( k ), liver ( l ), spleen ( s ). fig5 b . real - time us molecular imaging of esel expression in the mouse lower abdomen . separate 14 mhz ‘ cps - contrast only ’ and ‘ b - mode images ’ before ( ix , x , xiii , xiv ) and ≈ 30 min post ( xi , xii , xv , vxi ) particle administration . kidney ( k ), spleen ( s ). fig6 . baseline images ( before particle administration ) for fig4 b . fig7 . temporal expression of esel in the mouse heart . ( a ) esel mrna . each data point represents one animal . exponential line of best fit ± 95 % confidence interval ( ci ) is shown . ( b ) esel cell - surface protein . each data point represents mean ± 95 % ci for 5 , 5 , 4 , 5 mice at lps time 3 , 5 , 8 , 24 h , respectively . exponential line of best fit ± 95 % ci is shown . ( c ) esel mrna vs . esel cell - surface protein . fig8 . acoustic quantification of esel expression in the mouse heart . pearsons r : ( a ) wt =− 0 . 87 , ko =− 0 . 08 ; ( b ) wt = 0 . 84 , ko = not applicable ; ( c ) wt =− 0 . 78 , ko =− 0 . 05 ; ( d ) wt = 0 . 81 , ko = not applicable . solid circle = wt , open square = esel ko . error bar represents sd . mes - 1 monoclonal antibody ( mab ), a rat igg2a , k against mouse esel , 10 and its f ( ab ′) 2 fragment ( mes - 1 f ( ab ′) 2 ) were provided by dr d brown ( ucb celltech , uk ). af488 - mes - 1 ( mes - 1 labelled with 7 alexa fluor ® 488 fluorescence dye ) and reduced mes - 1 f ( ab ′) 2 ( 2 thiol ( sh ) groups per f ( ab ′) 2 from tris ( 2 - carboxyethyl ) phosphine hydrochloride ( tcep ) reduction ) were prepared as described below . mec13 . 3 mab , rat igg2a , k against mouse pecam - 1 ( bd biosciences ). allophycocyanin - labelled mab against mouse pecam - 1 ( bd pharmingen ). rat igg2a , k isotype negative control mab ( bd biosciences ). biotinylated rabbit mab against rat igg2a ( secondary antibody ) ( vector laboratories ). mes - 1 f ( ab ′) 2 was reduced using 4 molar excess tcep ( sigma - aldrich ): mes - 1 f ( ab ′) 2 ( 83 . 3 μm , 8 . 3 mg / ml ) and tcep ( 333 . 3 μm , 0 . 096 mg / ml ) in exchange buffer ( 50 mm 2 -( n - morpholino ) ethanesulfonic acid ( sigma - aldrich ), 2 mm ethylenediaminetetraacetic acid ( sigma - aldrich ), ph 6 ) were incubated for 1 h at 37 ° c . under constant agitation . the reaction volume ranged 1 - 1 . 6 ml . the reaction was stopped by placing on ice and immediate purification of the reduced f ( ab ′) 2 using spin column gel filtration chromatography with a 5 ml - zeba desalt spin column ( size exclusion limit 1 , 000 da ) according to the manufacturer &# 39 ; s instructions ( perbio science ), at 4 ° c . the spin column was previously equilibrated in cold exchange buffer . the degree of reduction of the f ( ab ′) 2 ( number of sh per f ( ab ′) 2 ) was determined spectrophotometrically using ellman &# 39 ; s test with ellman &# 39 ; s reagent ( perbio science ) according to the manufacturer &# 39 ; s instructions with the following modifications : the ellman &# 39 ; s reaction consisted of 2 . 5 μl ellman &# 39 ; s reagent ( 10 mm , 4 mg / ml ), 7 . 5 μl exchange buffer and 90 μl of the purified reduced f ( ab ′) 2 in exchange buffer , incubated at room temperature ( rt ) covered with aluminium foil to minimise light exposure ; absorbance at 412 nm ( a 412 ) was measured at 24 min from the start of the ellman &# 39 ; s reaction , to determine the concentration of sh in the reduced f ( ab ′) 2 sample by reference to a standard curve of ellman &# 39 ; s reaction with known concentrations of sh - containing compound , l - cysteine hydrochloride ( perbio science ) in exchange buffer ( ph6 ); duplicate ellman &# 39 ; s ‘ blank ’ reaction where the exchange buffer was added in place of the reduced f ( ab ′) 2 was used for baseline subtraction of a 412 from the test samples . the concentration of reduced f ( ab ′) 2 was determined from a 280 in the absence of ellman &# 39 ; s reagent ( with the spectrophotometer zeroed using exchange buffer ), as the latter would interfere with a 280 . the degree of f ( ab ′) 2 reduction was calculated as : the reduced f ( ab ′) 2 contained 2 sh groups per molecule of f ( ab ′) 2 . the purified reduced f ( ab ′) 2 was kept at concentration of ≈ 8 mg / ml ( 80 μm ) in exchange buffer for subsequent conjugation to microparticles . ( b ) labelling of mes - 1 mab with alexa fluor ® 488 ( af488 ) fluorescence dye for confocal microscopy . mes - 1 was labelled with alexa fluor ® 488 carboxylic acid , 2 , 3 , 5 , 6 - tetrafluorophenyl ester , 5 - isomer ( molecular probes ) as follows . a 100 μl - labelling reaction mixture consisting of 50 μm mes - 1 mab ( 25 . 7 μl at 194 μm or 29 . 1 mg / ml in phosphate buffered saline ( pbs ) ph 7 . 5 ), 750 μm af488 ( 6 . 6 μl at 11 . 3 mm or 10 mg / ml in water ), 2 . 6 μl ( 1 / 10 th volume of mes - 1 mab added ) 1m nahco3 ph 8 . 5 and 6541 distilled h 2 o , was incubated at rt for 1 h , with gentle manual agitation at 30 min . controls included no dye or no mes - 1 mab . af488 - labelled mes - 1 was then purified from the reaction mixture and suspended in pbs ( ph 7 . 5 ), using gel - filtration chromatography spin column with 6 kda size exclusion limit ( bio - spin p - 6 column with ssc packing buffer , biorad ) according to the manufacturer &# 39 ; s instructions with the following specific conditions : the bio - spin column was buffer exchanged first with pbs using 3 wash cycles ; all centrifugations were carried out at 20 ° c . the concentration and degree of labelling of af488 - labelled mes - 1 was determined using a spectrophotometer . samples were diluted in pbs to 100 μl volume in duplicates , and absorbance measured at 280 nm ( a 280 ) and 495 nm ( a 495 ). the concentration of the labelled mab was calculated as : where 0 . 11 is the correction factor for af488 &# 39 ; s contribution to a 280 . the concentration of the mab in mg / ml was converted to where 150 , 000 is the molecular weight of mab in da , 10 6 is the multiplication factor for converting m to μm . the concentration of af488 was calculated as : where 71 , 000 is the approximate molar extinction coefficient of af488 dye ( in cm − 1 m − 1 ) at 494 nm , 10 6 is the multiplication factor for converting m to μm . finally , the degree of labelling was calculated as : aluminium foil was used at all stages to minimise light exposure . under this protocol , unreacted af488 was retained in the column , confirmed by the ‘ no mes - 1 mab ’ control reaction . the labelling reaction molar ratio of 15 : 1 ( af488 : mes - 1 ) yielded mes - 1 labelled with 7 af488 molecules each , labelling efficiency = 46 %, yield ≈ 89 %. the purified af488 - labelled - mes - 1 ( 6 . 748 mg / ml , 44 . 984 μm ) was aliquoted , wrapped in aluminium foil and store at − 20 ° c . until use . the preservation of sensitivity and specificity to esel of the af488 - labelled mes - 1 was confirmed using immunohistochemistry on frozen heart sections of wild - type ( wt ) and esel knock - out ( ko ) mice pre - treated with lipopolysaccharide ( lps ). native ( unconjugated ) maleimide - functionalised lipid - shelled octafluoropropane ( c 3 f 8 ) microparticles were prepared by sonication of an aqueous suspension containing 1 , 2 - distearoyl - sn - glycero - 3 - phosphocholine ( dspc ; avanti polar lipids , al ), 1 , 2 - distearoyl - sn - glycero - 3 - phosphoethanolamine - n -( maleimide ( polyethylene glycol )- 2000 ) ( dspe - peg 2000 - mal ; avanti polar lipids ), mono - stearate poly ( ethylene ) glycol ( peg 40 stearate ; sigma - aldrich ), and fluorescent dye 1 , 1 ′- dioctadecyl - 3 , 3 , 3 ′, 3 ′- tetramethylindocarbocyanine perchlorate ( dii ; molecular probes ) at 75 : 9 : 14 : 2 molar ratio , in the presence of c 3 f 8 . to make esel targeting particles , tcep - reduced mes - 1 f ( ab ′) 2 containing 2 sh groups per f ( ab ′) 2 were grafted to the shell outer - surface of these native particles by maleimide - thiol conjugation . the conjugation reaction ratio was 4 . 338 × 10 6 f ( ab ′) 2 molecules per particle ( 7 . 2 nmol f ( ab ′) 2 per 10 9 particles ). ( nb . the total number of dspe - peg 2000 - mal molecules in the aqueous suspension / total number of unwashed particles produced from the aqueous suspension = 4 . 338 × 10 6 . if ≦ 10 % of the components in the aqueous suspension were incorporated onto the particle shell , and the molar ratio of the components on the shell remained close to that of the initial aqueous suspension , 56 then a particle of population mean size would contain ≦ 4 . 338 × 10 5 maleimide molecules , and the estimated f ( ab ′) 2 : maleimide conjugation reaction molar ratio would be ≧ 10 : 1 ). the conjugation reaction was carried out for 30 minutes ( min ) at 4 ° c ., near neutral ph , under c 3 f 8 atmosphere with constant gentle agitation ; the reaction was terminated by adding excess n - ethylmaleimide ( nem ) to quench any unreacted sh . particles were washed with cold degassed normal saline using multiple cycles of centrifugation flotation under c 3 f 8 atmosphere at 4 ° c . before and after particle conjugation , to remove unincorporated components and particle fragments . freshly prepared particles were immediately divided into 20 - 50 μl aliquots , capped and sealed with parafilm ( american national can ), then snap frozen in liquid nitrogen and stored at − 80 ° c . until use . the concentration of subsequently thawed esel targeting particles ranged 1 - 3 × 10 9 particles / ml amongst 5 batches prepared at different times . detailed experimental conditions are as follows . native ( unconjugated ) microparticles were prepared by dispersing dspc , dspe - peg 2000 - mal , peg 40 stearate and dii at molar ratio of 75 : 9 : 14 : 2 in a small amount of cyclohexane : chloroform solvent ( 1 : 2 ), in a 50 ml round - bottomed flask . excess solvent was extracted using a stream of gaseous nitrogen . the lipid was then transferred to a freeze dryer and lyophilised to full dryness under a reduced atmosphere ( e . g ., 1 . 3 × 10 4 pa ) at − 78 . 5 ° c . ( using a jacket of dry ice ). the dry powder ( lyophilisate ) was then dispersed in a suitable aqueous - based solution ( e . g ., normal saline or normal saline containing 0 . 01 % propylene glycol ( pgns : propylene glycol 103 . 5 mg / ml , glycerol 126 . 2 mg / ml , nacl 6 . 8 mg / ml , ph ≈ 7 . 4 )), to a concentration of 4 mg / ml , homogenised by sonication in an ultrasonic bath at 60 - 65 ° c . until transparent . once fully dissolved , the solution was gently sparged with c 3 f 8 gas ( f2 chemicals ). microparticles were then formed using a shear - mixing approach , by sonic dispersion of c 3 f 8 using a sonicator ( misonix 3000 , qsonica , ct ). the probe tip was positioned about 2 mm into the solution and sonication was performed with a high - intensity ultrasound horn ( 20 - 21 khz ) for 30 - 60 s at an acoustic power of approximately 120 w with an initial temperature of approximately 60 ° c . more c 3 f 8 gas was sparged into the microbubble dispersion , and the vessel capped and immediately plunged into ice cold water ( 3 min ) to dissipate the heat generated during the sonication process . microparticles produced were washed ( purified ) by centrifugation flotation at 1 , 000 g 4 ° c . for 15 - 25 min , using a beckman coulter allegra x - 15r centrifuge ( beckman coulter ): particles float to the top of the sample vial after centrifugation , the subnatent was removed and replaced with equal volume of cold degassed normal saline ( ph 7 . 4 ). the wash step was repeated 7 times to remove unincorporated shell components and particle fragments . to produce esel targeting microparticles , these washed native microparticles were added to reduced mes - 1 f ( ab ′) 2 whilst mixing ( each reduced f ( ab ′) 2 molecule contained 2 sh groups , prepared as described above ). the conjugation reaction ratio used to produce the successful esel targeting particles was 4 . 338 × 10 6 f ( ab ′) 2 molecules per particle . the concentration of particles and f ( ab ′) 2 in the conjugation reaction mixture ranged 5 - 8 × 10 9 / ml and 35 - 60 um ( 3 . 5 - 6 mg / ml ), respectively . the reaction mixture contained approximately ⅔ volume of exchange buffer ( ph 6 ) from the reduced f ( ab ′) 2 and ⅓ volume of normal saline ( ph 7 . 4 ) from the washed particles . the conjugation reaction was incubated at 4 ° c . for 30 min , continuously mixed gently on a vertically tilted rotating wheel . particle conjugation was terminated by adding 80 mm nem ( sigma - aldrich ) dissolved in dry dimethyl sulfoxide ( dmso , sigma - aldrich ) at 20 molar excess to f ( ab ′) 2 — the reaction mixture was incubated at 4 ° c . for 30 - 60 min on the rotator . typically , the concentration of nem and dmso in the reaction mixture was ≈ 1 mm and ≦ 1 . 7 % v / v , respectively . the particles were then washed 4 times with cold normal saline by centrifugation flotation as described above , at 160 g 4 ° c . for 5 min . this removed unincorporated f ( ab ′) 2 , unreacted nem , dmso and particle - fragments . to minimise particle loss , all washes and incubations were performed with the particle concentrations kept high (≧ 1 × 10 9 particles / ml ), under c 3 f 8 atmosphere ( to reduce the concentration gradient for diffusion of c 3 f 8 - gas out of the particles ) and at 4 ° c . ( to reduce the rate of gas diffusion ). to preserve the particles dii fluorescent dye , the dii compound , lyophilisate or particles were protected from light . particle integrity ( spherical morphology , absence of aggregation ) was examined under microscopy with the particles placed in a haemocytometer ( reichert bright - line metallized hemocytometer , hausserscientific , pa ) at e . g ., 1 : 200 dilution in cold normal saline . particle concentration and size distribution were determined by electrozone sensing in a coulter multisizer ile equipped with a 30 μm - diameter orifice counting tube ( coulter electronics ), according to the manufacturer &# 39 ; s instructions . the set - up allowed size detection range 0 . 72 - 18 μm , resolution 0 . 09 μm . for particle charge analysis , the particles were dispersed in 1 ml of 1 mm kcl ( ph 7 . 4 ) at ≈ 10 7 particles / ml . its net charge was determined as the zeta potential by light scattering in a zetasizer nano zs ( malvern instruments ), according to the manufacturer &# 39 ; s instructions . 100 μl of esel targeting or non - targeting ( native ) particles at 2 . 5 × 10 7 particles / ml were placed on inverted polystyrene petri - dishes coated with 200 μl of recombinant homodimeric mouse esel protein ( r & amp ; d systems ) at 7 nm ( dish e ), or on esel coated dishes previously blocked with 500 μl of excess mes - 1 f ( ab ′) 2 at 67 nm ( dish b ), or on non - coated dishes where phosphate buffered saline ph 7 . 5 ( pbs ) was used instead of esel for dish coating ( dish p ). unattached particles were gently washed off after 1 min . the dishes were then re - filled with cold degassed pbs for immediate examination under an upright light microscope equipped with immersion objective lens . the number of particles attached on each dish was counted and averaged from 10 random optical fields ( ofs ) to determine the attached particle density . the detailed methodology is as follows . polystyrene petri - dish ( corning ® 35 mm not tc - treated culture dish , corning life sciences ) was coated with 200 μl recombinant homodimeric mouse esel protein ( r & amp ; d systems ) at 1 . 25 μg / ml ( 7 nm , diluted in phosphate buffered saline ph 7 . 5 ( pbs )) for 1 h at rt ( dish e ). pbs was used instead of esel as ‘ blank ’ negative control ( dish p ). non - specific binding sites were then blocked with 4 ml bovine serum albumin ( bsa , sigma - aldrich ) at 2 . 5 % w / v in pbs for 2 h at rt . bsa was then discarded and the dish washed with pbs . to prepare esel coated dish blocked with excess mes - 1 f ( ab ′) 2 ( dish b ), 500 μl mes - 1 f ( ab ′) 2 at 6 . 67 μg / ml ( 67 nm ) was placed in dish e for 30 min at rt , then washed with pbs . due to the buoyancy of the particles , the dishes were inverted for incubation with particles for 1 min at rt : 100 μl esel targeting or non - targeting ( native ) particles at 2 . 5 × 10 7 particles / ml ( diluted in cold degassed pbs ) were used . unattached particles were then gently washed off and the dishes re - filled with cold degassed pbs for immediate examination using an upright light microscope equipped with immersion objective lens , connected to a camera and monitor . the number of particles attached to each dish was counted and averaged from 10 random optical fields ( ofs ) on the monitor display , the surface area of the latter determined using a stage micrometer . the attached particle density was thus determined . wild - type ( wt ) mice : adult male c57bl6 / jax ( charles river , uk ). esel knock - out ( ko ) mice : adult male esel homozygote ko on c57bl6 background , 57 bred locally from mice donated by dr k norman and prof p hellewell ( university of sheffield , uk ). all the animal work was carried out under project licenses and personal licenses granted by the home office under the animals ( scientific procedures ) act 1986 ; ethical approval was additionally obtained from the local ethical review panel . wt and esel ko mice were pre - treated with 50 μg lps from e coli 0111 : b4 ( sigma - aldrich ), made up to 200 μl volume in normal saline , by intraperitoneal ( ip ) injection to induce systemic inflammation . systemic administration of lps by ip injection produces systemic inflammation , which includes induction of esel expression in multiple organs including the heart and kidneys . 58 , 59 this animal model differs from others used in particle - targeting ( such as ischaemia - reperfusion injury , heart transplant rejection , thrombosis , chronic - ischaemia / tumour angiogenesis animal models ) in that : ( i ) it does not require surgical procedures which may introduce confounding variables such as surgical trauma or blood clots ; ( ii ) the particles target molecules present in multiple organs ( not just one ), uniquely allowing assessment of targeting particle specificity in multiple organs simultaneously , and assessing the imaging technique &# 39 ; s ability to detect target molecule in a tissue of interest in the presence of ‘ particle steal ’ by other tissues ; and ( iii ) the target molecule expression in the myocardium was essentially global and uniform , uniquely allowing the study of targeted particle signal attenuation . additionally , the combination of esel being solely expressed on activated endothelial cells and its essentially global uniform expression in the myocardium of lps pre - treated mice , makes the lps mouse model - esel combination an ideal in vivo model for testing acoustic quantification of molecular expression . immunohistochemistry was performed on acetone - fixed cryosections of freshly harvested hearts of wt ( with / without lps pre - treatment ) and esel ko ( pre - treated with lps ) mice . after blocking non - specific binding sites with 100 μl of 1 : 1000 rabbit serum ( sigma - aldrich ) for 1 hour ( h ) at room temperature ( rt ), sections were incubated for 1 h at rt with 100 μl of 0 . 01 mg / ml primary antibody : mes - 1 ( for esel ), mec13 . 3 ( for pecam - 1 , endothelial marker ) or isotype negative control . each section was then incubated with 100 μl of 0 . 005 mg / ml biotinylated secondary antibody for 60 min at rt . after blocking of endogenous peroxidase with 0 . 3 % h 2 o 2 methanol for 20 - 30 min at rt , the horseradish peroxidase - based detection system , vectastain abc kit ( vector laboratories ), was used with 3 , 3 ′- diaminobenzidine solution ( sigmafast ™ dab tablet , sigma - aldrich ) as the chromagen substrate . sections were counterstained using harris modified hematoxylin solution ( sigma - aldrich ) and 1 % nahco 3 , then dehydrated through 70 - 100 % ethanol , dried and mounted with histomount ( vwr ), and examined under light microscopy . the duration between the time of lps pre - treatment and sacrifice of the animal for immediate tissue harvesting was noted as the lps time . wt mice were pre - treated with lps as described above . the duration between the time of lps pre - treatment and sacrifice of the animal for immediate tissue harvesting was noted as the lps time . freshly harvested tissues were kept in rnalater ® solution ( ambion ) to preserve ribonucleic acid ( rna ) in - situ ; total rna was subsequently extracted using trizol reagent ( invitrogen ) according to the manufacturer &# 39 ; s instructions . first - strand complementary deoxyribonucleic acid ( cdna ) synthesis was then performed using the qiagen omniscript ® reverse transcription kit ( qiagen ) according to the manufacturer &# 39 ; s instructions . this was followed by real - time qpcr with the sybr ® green detection method for esel and hypoxanthine phosphoribosyltransferase - i ( hprt - i ), carried out on a 96 - well plate in the icycler ™ ( icycler iq real - time pcr detection system , bio - rad ) according to the manufacturer &# 39 ; s instructions . all pcr reactions were carried out in triplicate wells on the same plate . the primer sequences were : esel forward primer 5 ′- ctcattgctctacttgttgatg - 3 ′, esel reverse primer 5 ′- gcatttgtgttcctgattg - 3 ′, hprt - i forward primer 5 ′- attagcgatgatgaaccag - 3 ′, hprt - i reverse primer 5 ′- agtctttcagtcctgtccat - 3 ′. for data analysis , the threshold cycle ( ct ) was determined from the amplification plot using the icycler ™ iq optical system software version 3 . 0a ( bio - rad ). as pcr efficiency of the esel and hprt - i primer pairs differed by ≦ 5 % ( 93 ± 4 % and 92 ± 3 % ( mean ± sd ), respectively ; n = 4 each ), comparative ct method was used to estimate the amount of esel messenger ( mrna ) relative to that of hprt - i , using the formula : esel mrna (% hprt - i )= 2 − δct , where δct = ct esel − ct hprt - i , subscripts refer to the gene of interest . mean of the replicates was used and plotted against lps time for each animal . further details of the methodology are as follows . the yield of total rna from the mouse heart was typically ≈ 1 μg pure rna per 1 mg tissue , kept at concentrations over ≈ 1 mg / ml in molecular grade ( rnase - free ) h 2 o ( sigma - aldrich ). the rt reaction mixture for first - strand cdna synthesis consisted of 1 μg total rna , 2 μl 10 × buffer rt , 2 μl deoxyribonucleotide triphosphate ( dntp ) mix ( 5 mm each 2 ′- deoxyadenosine 5 ′- triphosphate ( datp ), 2 ′- deoxycytidine 5 ′- triphosphate ( dctp ), 2 ′- deoxyguanosine 5 ′- triphosphate ( dgtp ), 2 ′- deoxythymidine 5 ′- triphosphate ( dttp ), 1μ ( 4 units ) omniscript reverse transcriptase , 2 μl ( 1 μg ) oligo ( dt ) 12 - 18 primer ( invitrogen ) and molecular grade h 2 o made up to a total reaction volume of 20 μl , incubated for 1 h at 37 ° c . qpcr was carried out in a 25 μl - reaction volume in each well of a 96 - well 0 . 2 ml thin - wall pcr plate ( bio - rad ) covered with an optical quality sealing tape ( bio - rad ). the qpcr reaction mixture consisted of 5 μl cdna template ( 1 : 50 water dilution of the finished rt reaction ), 0 . 5 μl ( 10 μm ) each of the forward and reverse primer for the respective gene ( see text for primer sequence ; the primers were custom ordered from invitrogen ), 6 . 5 μl molecular grade h 2 o and 12 . 5 μl iq ™ sybr ® green supermix ( bio - rad ). the qpcr cycling condition was : initial 3 min denaturing step at 95 ° c . ( well factor analysis in first 90 s ); then 40 cycles of 15 s at 95 ° c ., 1 min at 56 ° c . ; melt - curve analysis in 0 . 5 ° c . steps ( 1 min denaturation at 95 ° c ., 1 min reset at 56 ° c ., then 80 cycles of 10 s at 60 ° c . with 0 . 5 ° c . increment for each cycle ); final cooling step at 4 ° c . esel and hprt - i were amplified on the same plate for each animal ; no - template negative control using molecular grade h 2 o in place of cdna template for both primer pairs were included in all plates . for data analysis , wells with abnormal amplification plot or melt - curve were excluded . inflammation of the cremaster muscle was produced by intrascrotal injection of 50 ng recombinant il - 1β ( r & amp ; d systems ) 2 h before surgery in wt and esel ko mice . the tail vein was cannulated with a 24 g 0 . 7 × 19 mm intravenous ( iv ) catheter ( dead space ≈ 50 μl ) ( bd medical ). long duration anaesthesia was achieved using intraperitoneal ( ip ) injection of 200 - 300 μl mixture containing 1 mg / ml xylazine ( rompum , bayer ) and 10 mg / ml ketamine hydrochloride ( ketalar , parke - davis ) in normal saline . the animal was placed on a custom - built thermo - controlled ( 37 ° c .) ivm stage . under a dissection microscope , the right or left testis was gently exteriorised through a scrotal incision . a longitudinal incision was made along the cremaster muscle , which was then spread out and pinned down across a translucent microscopy stage . the exteriorised muscle was maintained by continuous super - fusion of thermo - controlled tyrode &# 39 ; s salt buffer solution ( 9 . 6 g tyrode &# 39 ; s salts ( sigma - aldrich )+ 1 g sodium hydrogen carbonate , made up to 1 l volume with sterile distilled water ). observations and recordings were made using an upright microscope ( axioskop , carl zeiss ) equipped for bright field and fluorescence microscopy , with 20 × and 40 × immersion objective lens ( water achroplan , carl zeiss ), a charge coupled device ( ccd ) camera ( color chilled 3ccd camera with controller , hamamatsu phototonics ), a silicon intensifier target ( sit ) camera ( c - 2400 - 08 , hamamatsu photonics ), a monitor ( triton , sony ), a s - vhs recorder ( model ag 6730 svhs 625 , panasonic ) and a personal computer . to compare shear rates against particle attachment between wt and esel ko animals , an optical doppler velocimeter ( microcirculation research institute , texas a & amp ; m university , texas ), was used to measure microvascular centreline red blood cell velocities ( v rbc ) in 20 - 40 μm diameter venules of the wt and ko mice , before particle injection . 5 random vessel - segments were measured per animal . 1 . 5 × 10 7 esel targeting particles in 100 μl normal saline were administered as a rapid iv bolus through a tail vein catheter , immediately followed by a 100 μl normal saline flush , for intravital microscopy assessment in the cremaster of anaesthetised ( xylazine / ketamine mixture ip ) wt and esel ko mice . all animals were pre - treated with 50 ng recombinant il - 1β ( r & amp ; d systems ) intrascrotally , 2 hours ( h ) before exteriorization of the muscle , to induce cremaster inflammation . observations were made using an upright microscope equipped for bright field and fluorescence microscopy , with 20 × and 40 × immersion objective lens , ccd and sit cameras . to determine the shear rates against particle attachment , v rbc were measured in 20 - 40 μm diameter venules from 5 random vessel - segments per animal , using an optical doppler velocimeter before particle injection . all recordings were made using a s - vhs recorder and personal computer . all animals were sacrificed at the end of the experiment . see above for details of the set - up . blood flow and particles were assessed over several ofs encompassing a number of different vessels under bright field and fluorescence microscopy . the number of freely circulating - particles in an of on the monitor were counted over 10 s under fluorescence microscopy at 5 , 7 , 10 ± 15 min after particle injection . the accumulation of attached particles ( defined as not moving for & gt ; 3 s ) in an of field were assessed for up to 15 min post particle injection . the number of particles attached to 20 - 40 μm diameter venules were counted at ≈ 15 min after particle injection ( when freely circulating - particles were absent / minimal ) from 1 - 5 400 μm length segments per venule , 2 - 6 venules per animal . in some animals , the attached particles in the same of were followed - up for up to 90 min using intermittent combined bright field and fluorescence microscopy , looking for the presence / absence of transmigration into the tissue interstitium or cellular internalisation . for analysis , the density of attached particles was expressed as the number of particles per vessel surface area ( vsa ), where vsa ( mm 2 )= πd ( mm )× l ( mm ), d and l were the vessel segment diameter and length , respectively . the attached particle density for each venule was taken as the mean of its segments , and that for each animal was taken as the mean of its venules . shear rate was estimated from v rbc as : v b is the mean bulk velocity , α the factor converting v rbc to v b , taken as 1 . 6 for poiseuille flow in veins . 60 the shear rate for each animal was taken as the mean of the 5 random venule segments sampled . 1 . 5 × 10 7 esel targeting particles were administered to wt and esel ko mice pre - treated 2 . 5 h beforehand with 50 ng il - 1β intrascrotally , using methods described for ivm above . 15 min later , a rapid iv bolus of 150 μl cocktail containing 50 μg af488 - mes - 1 ( for esel ) and 25 μg allophycocyanin - labelled mab against mouse pecam - 1 ( endothelial marker ) in normal saline was administered iv , followed by a 100 μl normal saline flush . after a further 15 - 20 min , animals were given terminal anaesthesia using iv bolus of a xylazine / ketamine mixture , followed by perfusion with pbs to remove unattached particles and mab in the circulation . immediately thereafter , the cremasters were harvested and fixed in 4 % paraformaldehyde pbs for confocal microscopy ( z - stacked ). 3 different fluorescence , dii ( particles ), alexa fluor ® 488 ( esel ) and allophycocyanin ( pecam - 1 ) were scanned in series at each depth before moving on to the next depth in the z - axis , using an upright confocal laser - scanning microscope ( lsm 5 pascal , carl zeiss ) with a 40 × immersion objective lens . images were processed using zeiss lsm 5 image browser software . further details of the methodology are as follows . following the iv administration of esel targeting particles and antibody cocktail containing af488 - mes - 1 + allophycocyanin - labelled mab against mouse pecam - 1 , unattached particles and mab in the circulation were removed by perfusion with pbs . this was achieved by exposing the heart and upper abdomen by dissection . a snip incision was then made in the right atrium followed by injection of pbs into the lv cavity using a needle connected to a 20 ml - syringe . this allowed the pbs to perfuse the body , it and the blood left the circulation via the incision in the right atrium . adequate pbs perfusion was assumed when the liver turned pale due to the replacement of blood by pbs . the cremaster muscle was then harvested immediately , spread out and fixed in 4 % paraformaldehyde pbs solution for 30 min at rt , then placed in cold pbs at 4 ° c . for 5 min . aluminium foil was used to minimise light exposure to the tissue . fresh tissues were examined immediately under confocal microscopy ( z - stacked ), using an upright confocal laser - scanning microscope ( lsm 5 pascal , carl zeiss ) with a 40 × immersion objective lens ( water achroplan , carl zeiss ). 3 different fluorescence , dii for particles ( excite at 543 nm , detect at 560 - 615 nm ), alexa fluor ® 488 for esel ( excite at 488 nm , detect at 505 - 530 nm ) and allophycocyanin for pecam - 1 ( excite at 633 nm , detect at 650 nm ) were scanned in series at each depth before moving on to the next depth in the z - axis . images were processed using accompanying software , zeiss lsm 5 image browser . wt and esel ko mice were all pre - treated with lps , tail vein cannulated and anaesthetised with xylazine / ketamine mixture as described above . the chest , abdomen and pelvis were then shaved and the animal placed supine . ecg electrode pads ( ambu ® blue sensor p , ambu ) were applied to the paws and connected to the us machine ( acuson sequoia ® 512 us system , siemens , ca ) equipped with ‘ small animal ecg filter ’. a layer of warm gel ( gel for ultrasonic & amp ; electrical transmission , henleys medical ) was coupled between the skin and us transducer ( 15l8 - s linear array transducer , foot print 26 mm , siemens ). us settings used were : 14 mhz ( p14 mhz , spatial resolution ≈ 0 . 2 mm ) contrast pulse sequencing ( cps ) mode , transmission power 9 db giving low mechanical index ( mi ) 0 . 22 - 0 . 26 estimated by the scanner , dynamic range 55 db , time gain 0 %, cps gain 8 , fundamental 2d gain 15 db , colour map m : 3 ( particle signal presented in heated object scale (‘ cps - contrast only ’ images ), tissue signal in grey scale (‘ b - mode ’ images )), teq was not used . before particle injection , baseline parasternal short axis ( psa ) view at the papillary muscle level , parasternal long axis ( pla ) and apical 4 - chamber ( a4c ) views of the heart with and without ‘ regional expansion selection ’ ( res ; giving magnified images with enhanced resolution ) were recorded as 3 s - digital clips . thereafter , imaging was maintained in the psa view with the transducer fixed in position using a free standing clamp . a stopwatch was then started and 1 × 10 8 esel targeting particles ( in 100 μl volume made up with normal saline ) injected at 10 s via the tail vein catheter as a rapid iv bolus over 1 - 2 s , followed by a 100 μl - normal saline flush over 1 - 2 s at 20 s . continuous us insonation was applied without pausing from time 0 - 1 min 23 s on the stop - watch , then paused , then resumed only for 3 s each time for digital image acquisition . 3 s - digital clips ( res activated ) of the heart containing several consecutive cardiac cycles were recorded at 10 s and 13 s , then at 10 s intervals from 20 s - 1 min 20 s , then at 1 min intervals from 2 min 20 s - 10 min 20 s , then at 2 min intervals from 12 min 20 s - 30 min 20 s , then at 5 min intervals until 60 min 20 s on the stopwatch ( image acquisition was stopped earlier if particle contrast enhancement in the left ventricular ( lv ) cavity ( central blood pool ) was no longer visible ). unmagnified ( non - res ) images of the thorax containing the heart in the psa view and surrounding tissues were recorded at ≈ 5 min intervals . other views of the heart ( pla and a4c views ) were acquired at the end . in some animals , 7 mhz ( p7 mhz , spatial resolution ≈ 0 . 4 mm ) cps imaging at mi 0 . 22 ( gain and other settings kept the same as 14 mhz imaging ) was also acquired at baseline and the end of the 14 mhz imaging study . when switching from 14 mhz to 7 mhz cps imaging , the transmit power was first reduced from − 9 db to − 19 db before reducing the us frequency , to avoid an increase in mi ( up to ≈ 0 . 7 ) causing inadvertent particle destruction . in some animals , imaging of other tissues in the thorax , abdomen and pelvis were performed in the antero - posterior projection at baseline and at the end of the cardiac imaging study , to look for particle retention in extra - cardiac tissues . to do this , the probe was positioned transversely and moved slowly caudally from just below the neck to the pelvis during 3 s - image acquisitions ; non - res images in 14 and 7 mhz cps were acquired . all animals received only one dose of particles to avoid carry - over effect from previous particle dosing ( e . g ., blocking of esel binding sites ). at the end of imaging , animals were sacrificed and tissues immediately harvested for frozen section immunohistochemistry and qrt - pcr as described above . videodensitometric method was used to quantify particle signal intensity off - line , using the yabco © software ( llc charlottesville , va .). end - diastolic image frames of the heart in the psa view (‘ cps - contrast only ’ images ) were selected and aligned , those that could not be aligned ( e . g ., due to large movement artefact ) were excluded . regions of interest ( rois ) were placed on the mid - anterior wall of the myocardium ( m ) and adjacent region in the lv cavity ( c ), as shown in fig4 a . these regions were chosen because they were consistently least or minimally affected by us attenuation in all animals . the video signal intensities ( vi ) were ‘ linearised ’ by log - decompression using the formula : linearised where db is the dynamic range ( 55 db in this study ). linearised vi ( i ) was expressed in arbitrary acoustic units ( au ). i of several end - diastolic image frames within the 3 s - recording period at each time point were averaged , then corrected for background noise by subtracting away average i of the baseline images ( images before particle administration ) in the respective animals . tics of the myocardium ( tissue ) and lv cavity ( central blood pool ) were constructed by plotting background - subtracted i of the myocardium and lv cavity , respectively , against time post particle administration . baseline - subtracted bubble signal intensity in the myocardium at 20 min 10 s post microparticle administration ( r20 ), obtained as per tic above , was used for analysis . an alternative novel method based on quantitative analysis of the tic ( tic - based method ) was also used to quantify the level of esel expression . the tic - based method also allowed other variables to be determined simultaneously , including the retained - and circulating - microparticle half - life in vivo . the lps time for us molecular imaging was taken as the duration between the time of lps pre - treatment and administration of the targeting microparticles . the mean heart rate ( hr ) for each animal was calculated from all hrs recorded at different time points during cardiac imaging . pearson correlation , linear or non - linear regression analysis was performed as indicated . student &# 39 ; s t - test or anova with turkey &# 39 ; s post - hoc analysis was used for significance testing where indicated , with p & lt ; 0 . 05 taken as significant . frozen section immunohistochemistry showed that esel was expressed in the heart of all wt mice pre - treated with lps ( n = 35 , lps time = 4 - 9 h ). the spatial distribution was essentially uniform throughout the myocardium but limited to the post - capillary venules and capillaries . esel was not detectable in the negative controls : wt mice not treated with lps ( n = 5 ), and esel ko mice pre - treated with lps ( n = 10 , lps time = 4 - 7 h ), fig1 . rt - qpcr showed that the concentration of esel mrna in the heart decreased exponentially with time after ≈ 3 h post lps pre - treatment , reaching very low levels by ≈ 9 h ( n = 42 , lps time = 3 . 2 - 15 . 7 h ), fig7 a . this trend was similar to that of the cell - surface esel protein concentration ( expressed as % injected dose of radioactivity / g tissue (% id / g tissue )) determined using iv radio - labelled mab by eppihimer et al . 58 using the same strain , sex , and age of mice , as well as the same dose and route of lps administration ( n = 19 ), fig7 b . from the best - fit curves of esel mrna concentration vs . lps time ( mrna ( in % hprt - i )= 3600e − 0 . 7lps time ( in hours ) , r 2 = 0 . 75 ) and that of esel cell - surface protein concentration vs . lps time ( protein ( in % id / g tissue )= 1 . 21e − 0 . 13lps time ( in hours ) + 0 . 07 , r 2 = 0 . 88 ), using lps time as the common denominator , an empirical formula describing the relationship between the concentration of esel mrna and cell - surface protein could be derived as : fig7 c . thus the esel cell - surface protein concentration was predictable from its mrna concentration in the heart , in this lps mouse model . within the mrna or protein concentration range ( 55 - 238 % hprt - i or 0 . 63 - 0 . 80 % id / g tissue , respectively ) used for us quantification of esel expression in this study ( see later ), the relationship between the concentration of mrna and cell - surface protein was approximately linear , allowing direct use of the mrna concentration as a surrogate quantifier for the cell - surface protein concentration ( the latter being the actual target of the targeting microparticles ). a schema of the esel targeting particles engineered is shown in fig2 a . these particles showed spherical morphology ; particle - particle aggregation or cross - linkage was minimal , fig2 b . the particle size distribution was reproducible amongst 5 batches prepared on separate occasions ; particle diameter = 2 . 2 ( mean )± 0 . 2 ( sem ) 98 . 6 % or 100 % of the particles were under 6 or 10 μm in diameter , respectively , fig2 c . the particles ( washed ) had a near neutral charge , zeta potential approximately 5 mv at ph 7 . 4 . the particles were sufficiently echogenic , stable , lacked non - specific binding , and produced no immediate adverse effects in vivo . particles made using other compositions did not exhibit all of these desired properties . the suitable native particles also contained enough maleimide groups for conjugating sufficient targeting elements onto the particle surface for efficient target binding under flow conditions . the conjugation reaction ratio used to produce the successful esel targeting particles was 4 . 338 × 10 6 f ( ab ′) 2 molecules per particle . lower f ( ab ′) 2 : particle reaction ratio 1 × 10 6 : 1 produced particles that could only attach to target under static conditions . it was also found that by keeping the conjugation sites on each binding element to a minimum ( e . g ., reducing only 1 interchain disulfide bond to produce 2 sh per f ( ab ′) 2 ), and inactivating any unreacted ones after particle conjugation ( e . g ., alkylation of unreacted sh ), significant particle aggregation due to cross - linking could be avoided . particles targeting other molecules can be readily made in the same way by substitution with suitable targeting ligands . the use of f ( ab ′) 2 instead of whole mab in this case eliminated any fc - medicated non - specific interaction or immunogenic side effects . the site - directed maleimide - thiol conjugation of targeting ligands to particles described herein is a departure from the immunogenic ( strept ) avidin - biotin conjugation chemistry traditionally used in preparing targeting particles . although other conjugation chemistries can be used , the site - directed maleimide - thiol conjugation method was advantageous due to its low immunogenicity , strong and rapid thioether bond formation at near neutral ph ( bond strength in the order of nanonewtons ; second - order rate constant 0 . 8 - 1 × 10 4 m − 1 s − 1 ). the near neutral ph was advantageous in avoiding negative impact on the binding elements and particles during preparation , and prevents dissociation of the binding elements from the particle shell in vivo . targeting particles based on similar conjugation chemistry , including non - covalent conjugation of targeting ligands to a phospholipid species before particle assembly , exist but only a few are progressing to ultrasound molecular imaging in vivo , 44 - 47 , 61 - 64 none of which showed all of the following desirable attributes which differentiate our microparticles from them : ( i ) high target binding specificity with proven minimal non - specific retention in remote non - res tissues not expressing the target molecule , following iv administration ; ( ii ) effective for real - time us molecular imaging ; and ( iii ) effective for acoustic quantification of molecular targets to a high quantitative degree . esel targeting particles attached to esel on coated dish ( dish e ): 2060 ( mean )± 1070 ( sem ) particles / mm 2 , n = 5 , fig2 d . specificity of the targeting particles against esel was demonstrated by their minimal attachment to esel previously blocked with excess anti - esel antibody ( dish b ): 8 ( mean )± 4 ( sem ) % of targeting particles on dish e , n = 5 . additional negative controls using targeting or non - targeting ( native ) particles on dish not coated with esel ( dish p ), or non - targeting particles on dish e and b showed similar low levels of particle attachment : targeting particles on dish p ( 9 ± 3 %, n = 5 ), non - targeting particles on dish e ( 10 ± 9 %, n = 2 ), b ( 7 ± 7 %, n = 2 ) and p ( 7 ± 7 %, n = 2 ). anova with turkey &# 39 ; s post - hoc analysis showed significant differences in the relative density of attached particles between targeting particles on dish e and the negative controls ( p & lt ; 0 . 0001 ); no significant difference was observed amongst the negative controls . esel targeting particles were administered to 5 wt ( body weight : 25 ( mean )± 2 ( sd ) g , range 23 - 27 g )) and 5 esel ko ( 24 ± 2 g , range 22 - 27 g ) mice at 3 : 06 - 4 : 03 h and 3 : 17 - 4 : 30 h post il - 1β pre - treatment , respectively . the particles were seen to circulate and reach the cremaster muscle ≈ 7 - 17 s post iv bolus administration through the tail vein . the number of freely circulating - particles decreased with time , their clearance from the blood pool occurred sooner in the wt than kos , fig3 a . beyond 10 min after particle administration , the number of freely circulating - particles in the blood pool was minimal or undetectable in all animals . the particles attached and accumulated on the cremaster venular wall of the wt animals ( 370 ( mean )± 46 ( sem ) particles / mm 2 vsa , n = 5 animals ); this was minimal in the kos ( 11 ± 3 , n = 5 ), p & lt ; 0 . 0001 ( student &# 39 ; s t - test ), fig3 c - d . particle accumulation on the cremaster vessel wall plateaued soon after particle - bolus administration , fig3 b . the accumulated particles persisted beyond the clearance of freely circulating ones from the blood pool . rolling was observed in a small minority of particles ; complete detachment of the attached particles was infrequently seen . intravascular obstruction by the particles was not observed . transmigration into the tissue interstitium or cellular internalisation of the attached particles were not detected , when observed intermittently for up to 90 min . confocal microscopy showed that the attached particles co - localised with esel expressed on the endothelial cell surface of the wt cremaster venule ( n = 3 animals , body weight 21 . 8 - 24 g ); esel expression was not detectable in the kos ( n = 2 , 25 . 7 - 28 . 4 g ), fig3 e . shear rates in the 20 - 40 μm diameter venules , determined from 3 wt ( body weight : 26 ( mean )± 2 ( sd ) g , range 25 - 28 g ) and 3 ko ( 27 ± 1 g , range 26 - 29 g ) mice , were higher in the wt ( 329 ( mean )± 46 ( sem ) s − 1 , n = 3 ) than ko group ( 211 ± 37 s − 1 , n = 3 ), probably due to smaller vessel diameters sampled in the former ( wt 30 ( mean )± 3 ( sem ) μm , n = 3 ; ko 34 ± 1 μm , n = 3 ). however , these differences were not statistically significant , p = 0 . 30 and 0 . 12 ( student &# 39 ; s t - test ) for shear rate and vessel diameter group comparison , respectively . 15 wt ( age 5 . 7 ( mean )± 0 . 2 ( sd ) weeks , range 5 . 1 - 6 . 1 weeks ; body weight 19 . 5 ( mean )± 1 . 5 ( sd ) g , range 17 - 22 g ) and 8 esel ko ( age 7 . 9 ± 3 . 2 weeks , range 5 - 13 . 6 weeks ; body weight 22 . 3 ± 3 . 7 g , range 17 - 28 g ) mice were imaged . the duration between lps injection and administration of targeting particles ( lps time ) ranged 3 : 26 - 5 : 59 h for the wt and 4 : 27 - 5 : 39 h for the ko group . non - linear cps artefacts ( orange colour ) present both before and after particle administration could be seen in wt and esel ko animals ; these artefacts were small and outside the myocardium , fig4 a ( frame 0 : 10 ), fig4 a 2 ( frame 20 : 20 ). following iv particle - bolus administration , particle signal was first detected in the right heart chambers within 4 heart beats (≈ 1 s ), the signal intensity rose rapidly . this was followed by the appearance of particle signal in the left heart chambers as the particles returned from the pulmonary circulation . particle signal intensity in the lv cavity and myocardium peaked within 6 - 7 heart beats (≈ 1 . 5 - 2 s ) and 9 - 12 heart beats (≈ 2 - 3 s ) after particle administration , respectively . particle signal intensity then decreased in the heart chambers and myocardium over time , fig4 a . significant us attenuation due to high particle concentration occurred early following particle - bolus administration , with major loss of signal in regions of the heart located distally in the us path ( e . g ., 5 - 10 o &# 39 ; clock positions in the myocardium and adjacent lv cavity in the psa view , fig4 a ). as the circulating - particle concentration in the blood pool ( lv cavity ) decreased over time , the attenuation diminished ( compare frame 0 : 30 vs . 6 : 20 , fig4 a ) but did not disappear ( frame 20 : 20 , fig4 a 1 )— most likely due to attenuation caused by overlying bone , lung air ± retained - particles . importantly , esel expression in the myocardium was visualised in real - time , indicated by the persistence of particle signal in the wt myocardium as the freely circulating - particles in the blood pool ( lv cavity ) decreased and disappeared over time . in the ko myocardium , the persistence of particle signal was low / minimal , consistent with a low / minimal degree of non - specific particle retention in the myocardium , fig4 a - b . us attenuation caused by overlying bone , lung air ± retained - particles located proximally in the us path , affected certain parts of the myocardium depending on the imaging scan plane — this caused pseudo - loss of targeted particle signal for esel in the wt animals ( e . g ., there was artefactual loss of retained - particle signal in the mid - posterior , - inferior , - posteroseptal and - anteroseptal walls of the lv ( anti - clockwise from 5 - 10 o &# 39 ; clock positions , respectively ) in the psa view with 14 mhz cps imaging ). however , by changing the scan plane ( e . g ., from psa to pla or a4c view ) to alter the relative position of overlying entities , or by lowering the us frequency to increase its penetrative depth ( e . g ., from 14 to 7 mhz ), these attenuation effects could be overcome with good recovery of the retained - particle signals , fig4 b . the global expression of esel in the wt myocardium was thus demonstrated on us imaging , consistent with the immunohistochemistry data . in the above situation , us detection of molecular target was limited by late distal attenuation from overlying bone / air and retained - particles located proximally in the us path . however , it was found that such attenuation could be overcome by using lower frequency us ( greater penetrative depth ) or a different imaging angle . from the human imaging perspective , where the use of lower frequency us ( e . g ., 3 - 7 mhz ) and multi - plane imaging are the norm , and the footprint of the transducer is much smaller relative to the body size ( making it easier to achieve optimal probe position / angle and avoid overlying bone / air ), these attenuation issues are likely less important . nevertheless , refinements in the machine &# 39 ; s attenuation correction algorithm may further minimise the attenuation artefacts . comparison of the thoracic , abdominal and pelvic scans between the wt and esel ko animals with reference to the baseline images ( before particle administration ), showed that non - specific retention of the targeting particles in non - res tissues was low / minimal . esel expression was detected in the renal cortex of wt but not ko animals ( fig5 a & amp ; 5 b ). immunohistochemistry and confocal microscopy confirmed that esel was expressed predominantly on endothelial cells of renal glomeruli , the latter concentrated in the renal . as expected , the targeting particles were taken up by the spleen and liver in both wt and ko animals ( fig5 a & amp ; 5 b ), the major res tissues involved in particle elimination . no death or significant adverse events attributable to the esel targeting particles were observed . no significant particle - medicated intravascular obstruction in the myocardium , causing loss of regional myocardial perfusion manifest as regional wall motion abnormality , was detected . ultrasound tic of the myocardium ( tissue ) and lv cavity ( central blood pool ) three phases with distinct characteristics were discernible from the tics following iv bolus administration of the targeting particles , fig4 a : ( 1 ) bolus - phase ( b ): lasting a few seconds , characterised by an initial rapid rise in particle signal intensity reaching peak or saturating / attenuation levels ( as particle concentration increased ) within seconds of bolus injection . ( 2 ) distribution - phase ( d ): lasting up to 1 - 2 min ( takes a few circulatory cycles to complete ), characterised by a short rapid decrease in particle signal intensity , as particles dilute by mixing and distribution to tissues . high particle concentration resulted in signal saturation / attenuation , obscuring re - circulation peaks . as the particle concentration decreased further over time , the particle signal intensity was frequently observed to paradoxically increase as attenuation decreased , giving rise to a second lower peak within 0 . 5 - 1 min of particle administration ( arrow in insets of fig4 a ). ( 3 ) elimination - phase ( e ): lasting several minutes , characterised by a long slow decrease in particle signal intensity . particle signal intensity in the lv cavity ( representing freely circulating - particle concentration in the central blood pool ) became undetectable sooner in the wt than esel ko animals ; in both groups they were essentially undetectable by 20 min post particle administration . in the myocardium , particle signal intensity of wt animals decreased slower than that of kos , and persisted beyond the disappearance of particle signal in the lv cavity . in ko animals , particle signal in the myocardium became undetectable before the disappearance of particle signal in the lv cavity ( as expected for relative myocardial blood volume ( rmbv )≦ 24 %), except when a detectable degree of non - specific particle retention was present . 12 wt ( age 5 . 7 ( mean )± 0 . 3 ( sd ) weeks , range 5 . 1 - 6 . 1 weeks ; body weight 19 . 7 ( mean )± 1 . 4 ( sd ) g , range 18 - 22 g ) and 8 esel ko ( age 7 . 9 ± 3 . 2 weeks , range 5 - 13 . 6 weeks ; body weight 22 . 3 ± 3 . 7 g , range 17 - 28 g ) mice were used . lps time ranged 3 : 53 - 5 : 59 h for the wt and 4 : 27 - 5 : 39 h for the ko group . excluded from quantitative analysis here were 3 wt animals because of : ( i ) microparticle dosing error ( 1 animal ); ( ii ) uncertainties regarding esel expression level ( 2 animals ); and ( iii ) 1 of these 3 animals has a tic that could not be adequately quantified , possibly due to severe attenuation artefact . in both wt and esel ko groups , freely circulating - particle signal in the blood pool ( lv cavity ) was absent / minimal by ≈ 20 min following iv bolus administration of 10 8 particles . therefore , background - subtracted i in the myocardium at 20 min 10 s post bubble administration ( r20 ) was used to represent that of the retained - particles only , as signal contribution from any residual freely circulating - particles by this time was negligible due to their low concentrations in the central blood pool ( signal intensity in the lv cavity ). e . g ., 0 . 1au in the lv cavity would contribute only 0 . 005 - 0 . 024au in the myocardium assuming a relative myocardial blood volume (% of myocardium that is blood ) of ≈ 5 - 24 %. 65 - 67 r20 was significantly higher in the wt ( 0 . 46 ± 0 . 16 ( sem ), range 0 . 01 - 1 . 61 , n = 12 ) than ko ( 0 . 06 ± 0 . 03 , − 0 . 01 − 0 . 24 , n = 8 ) animals , p & lt ; 0 . 05 . r20 correlated strongly with lps time in the wt ( r =− 0 . 78 , p & lt ; 0 . 05 , n = 12 ) but not ko mice ( r =− 0 . 05 , p = 0 . 9 , n = 8 ), fig8 c . it also correlated strongly with the concentration of esel mrna ( r = 0 . 81 , p & lt ; 0 . 005 , n = 12 ), fig8 d . the maximum retained - particle signal intensity in the myocardium , a r , was significantly higher in the wt ( 2 . 3 ± 0 . 4 ( sem ), range 0 . 8 - 4 . 2 , n = 12 ) than esel ko ( 0 . 4 ± 0 . 1 , 0 - 1 , n = 8 ) animals , p & lt ; 0 . 005 . the low a r values in the kos suggested minor degrees of non - specific particle retention . a r correlated strongly with lps time in the wt ( r =− 0 . 87 , p & lt ; 0 . 0005 , n = 12 ) but not ko mice ( r =− 0 . 08 , p = 0 . 86 , n = 8 ), fig8 a . by converting lps time to the concentration of esel mrna using the curve in fig7 a , fig8 b showed that a r correlated strongly with the concentration of esel mrna ( r = 0 . 84 , p & lt ; 0 . 001 , n = 12 ), the latter in the range that was approximately linearly related to the concentration of the cell surface esel protein , fig7 c . the half - life of the freely circulating - particles in vivo was ≈ 2 ( mean )± 1 ( sd ) min , range 1 - 5 min for the wt ( n = 12 ) and ko ( n = 8 ) animals . this was comparable with most commercially available non - targeting microbubbles ( range ≈ 1 - 3 min ), demonstrating that the microparticles produced were of commercial quality or better . the elimination of the retained - particles in the myocardium decreased with increased maximum concentration of retained - particles in the myocardium . the relationship was non - linear and could be empirically fitted to an exponential or sigmoidal function . this resulted in the half - life of the retained - particles being shorter the lower the maximum retained - particle concentration . the in vivo half - life of the acoustically effective retained - particles in the myocardium in these groups of animals was ≈ 7 ( mean )± 5 ( sd ) min , range 3 - 18 min in the wt ( n = 12 ) compared with 4 ± 4 min , range 1 - 14 min in the ko animals ( n = 8 ). the accumulation of attached microparticles on the target surface was essentially complete soon after particle - 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