THE UNIVERSITYOF NEWMEXICOHEALTHSCIENCESCENTER

COLLEGEOF PHARMACYALBUQUERQUE,NEWMEXICO

‘l’he university of New Mexico

Correspondence Continuing Education Coursesfor

Nuclear Pharmacistsand Nuclear MedicineProfessionals

VOLUMEVII, NUMBER6

An Update on the Detection ofDeep Vein Thrombosis

By:

PeterEu, MSC,RPhRadiophamcist

Departmentof NuclearMedicineand PET CentrePeterMacCallumCancerInstitute

12CathedralPlaceEast Melbourne,Victoria3002

AUSTRALIA

m The Univcmity of New Mexico Hcalih Sciences Cenler C’nlltge of Pharmacy is approved hy the American Councd on Pharmaceutical

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CO~ri@l I 999Univtrs!ty of New Mexico IIealth Sciences Center

Phtirmacy Continuing EducationAlhuqucrquc, New Mtxico o

AN UPDATE ON THE DETECTION OFDEEP VEIN THROMBOSIS

STATEMENT OF OBJECTIVES

The purpose of this lesson is to provide a general review of thrombus formation and anoverview of techniques for the detection of deep vein thrombosis (DVT). Specifically,this unit will review the potential problems associated with thrombus formation anddifficulties that may arise in achieving an accurate diagnosis. It also compares andcontrasts established detection techniques with the use of recently approvedradiopharmaceutical agents in the diagnosis and localization of DVT.

Upon completion of this continuing education unit, the reader should be able to:

1. Discuss the etiology of thrombus formation.

2. Describe the relationship between DVT and pulmonary embolism.

3. Discuss signs and symptoms of DVT.

4. Discuss the rationale used in the development of medical imaging techniques forthrombus localization.

5. Outline the advantages and disadvantages of contrast venography.

6, Describe the role of impedance plethysmography in the detection of thrombosis.

7. Discuss the principle of compression ultrasound in detecting DVT.

8. Summarize the role of magnetic resonance imaging in the detection of DVT.

9. Discuss the pharmacoeconomic impact of radiopharmaceutical agents in the detectionof DVT.

COURSE OUTLINE

1.

11.

III

Iv.

v.

VI.

v*

INTRODIJCTION

ETIOLOGYA, TraumaB. Venous Stasisc. Hypercoagulopathy

THROMB!JS EVENTS

PROPERTIES OF AN IDEALDVT DETECTIONPROCEDURE

DETECTION TechniquesA, Contrast VenographyB. Impedance

Plethysmographyc. UltrasoundD. Magnetic Resonance

Imaging

NIJCLEAR MEDICINEIM.AGING TECHNIQUESA. Radionuclide VenographyB. Platelet Scintigraphyc. Fibrinogen Uptake and

Fibrinogen ScintigraphyD. Immunoscintigraphy

1. Monoclinal Antibodies2. Synthetic Peptides

E. DisintegrinsF. Recombinant Tissue

Plasminogen Activator

SUMMARY

AN UPDATE ONTHE DETECTION OF

DEEP VEIN THROMBOSISBy:

Peter Eu, MSC, RPhRadiopharmacist

Department of Nuclear Medicineand PET Centre

Peter MacCallum Cancer InstituteEast Melbourne, Victoria 3002

AUSTRALIA

●

INTRODUCTION

Thrombosis is the formation,development or existence of a blood clotor thrombus within the vascular system.This can be a life-saving process when itoccurs during a hemorrhage. However, itbecomes a life-threatening event when itoccurs at any other time because the clotmay occlude a vessel and stop the bloodsupply to an organ or other body part.The thrombus, if detached, becomes an @

embolus and may occlude a vessel atsome distance from the original site (e.g.,a clot formed in the leg may break offand travel to the lungs causing apulmonary embolus).

The venous system, especially inthe deep veins of the lower limbs, is themost common site of thrombosis, due tothe relatively slow blood flow. Thethrombus fo~med in slow-moving bloodis comprised of a layer of plateletaggregates alternating with a fibrin

network containing leukoc~es and redblood cells. 1

Although deep vein thrombosis(DVT) is a common clinical event, itremains a difftcult diagnostic problem.The diagnosis and prompt treatment ofDVT is of notable importance in theprevention of pulmona~ embolism, aleading cause of morbidity and mortality.

o

1

● Pulmonary embolism is a oftencomplication of DVT and almost alwaysoccurs as a result of venousthrombosis.253 Available data indicate thatmore than 90°/0 of pulmonary emboliarise from thrombi in the venous systemof the lower extremities.4 The incidenceof pulmonary embolism in conjunctionwith proven cases of DVT is 30°/0 to43Y0.5

Diagnosis of DVT based onclinical signs and symptoms alone isboth non-specific and insensitive. DVTfrequently are present in the absence ofclinical signs and are absent in as many as50% of patients in whom clinical signsand symptoms suggest their presence.dThe clinical features of DVT arise fromreactive inflammation, venousobstruction, and diversion of bloodthrough the superficial veins. Thesefeatures include pain, swelling, redness,

9

warmth, tenderness, edema andprominent superficial veins.b None ofthese signs and symptoms are unique toDVT. In addition, various other

disorders such as muscle strain, cellulitisor ruptured Baker’s cyst can mimic thedisease,’ Clinical diagnosis is alsocomplicated by non-obstructive thrombiwith minimal symptoms such as thoseoccurring post operatively.

Venous thromboembolism is amajor cause of death and morbidityamong hospitalized patients. The disease,however, is often clinically silent withpulmonary embolism going undetected inas many as 70°/0 to 80°/0 of the patientswhose conditions are not diagnosed untilautopsy, Therefore, it is suspected thatthe true incidence of venousthromboembolism may be significantlygreater than actually quoted. This findingmay be related to the relatively low rate

oof autopsies performed in non-acute carefacilities such as rehabilitation hospitals

and nursing homes where the incidenceof pulmonary embolism may be higher,

10

Although the signs and symptomsof DVT are absent in approximately 50°/0of the patients, they range from subtle toobvious in the remaining 50°/0 of cases.The lack of objective clinical findingscreates much difficulty in the diagnosis ofDVT. At best, clinical examination aloneindicates DVT in only 50°/0 of cases,while falsely suggesting the diagnosis(false-positive results) in 30% to 60% ofcases. 11 Once the clinician is alerted tothe possibility of DVT, variousdiagnostic procedures may be carriedout. Accurate diagnosis, however, isfacilitated by an understanding andappreciation of the most common sites ofthrombus formation, the likelihood ofpropagation, the type of patients at highrisk, signs and symptoms, and propertesting.

ETIOLOGY

Risk factorsthrombus formationespecially followingparturition, cardiac

associated withinclude trauma,

an operation orand vascular

disorders, obesity, genetic predisposition,increasing age, an excess of erythrocytesor platelets, an overproduction offibrinogen, and sepsis.

The three major recognizedfactors in the etiology of DVT are: localinjury to vessels (injured endothelium),venous stasis or turbulence of bloodflow, and alterationof the blood itself

Trauma

(hypercoagulability)

Injury to the endothelial surfaceof the vessel results in platelet and fibrinadherence to the subendothelial collagen.Release of adenosine diphosphate (ADP)

2

from damaged endothelial cells and redblood cells, and adherent platelets,augments the buildup of the plateletclump. This initial aggregation ofplatelets is reversible. However,irreversible aggregation quickly occursdue to thrombin formation through theactivation of factor XII by exposedcollagen (intiinsic pathway) and therelease of thromboplastin from injuredendothelium (extrinsic pathway),

Venous Stasis

Another leading cause of DVT isvenous stasis. Venous stasis may becaused by factors such as prolongedimmob-ilization, debilitating medicalconditions, stroke, myocardial infarction,heart failure, obesity, varicose veins,anesthesia, and age greater than 65 years.

Hypercoagulopathy

Hypercoagulopathy also plays arole in the development of DVT.Hypercoagulation can be caused by avariety of factors including the following:hypervimsity, increased plateletadhesiveness, malignant disease, highlevels of estrogen, thrombocytosis,antiphospholipid syndrome, increasedclotting factors, and increased levels offibrinogen.g

THROMBUS EVENTS

A venous thrombus formationmay be clinically asymptomatic, undergospontaneous lysis, alter the venouscirculation and cause symptoms. lt alsomay propagate and possibly extend intothe more proximal veins, embolize, orany combination of the aforementionedevents may occur.

thrombus in thit is highly likepart thereof) wendothelium. ~time during t]may resultpulmonary sysl

Chroniusually incorp~veins. Long-telpossible veno~development 01

PROPERTIEDETECTIOh

The o]detection of clithe use of a diboth sensitive zlocations of thlthe legs. rdifferentiate btthrombus, parimportant condecisions.s Anavailable andprompt, reliaminimal discortest also shoureproducible. ,can result inrisks of anti-cnegative diagno!of clot propagati

Whileprocedures cuthe diagnosipresent, no sirthe above triteavailable has 1challenge is toimaging methclimitations of zis capable c

3

contribution to the clinical andtherapeutic management of this group ofpatients.

DETECTION TECHNIQUES

Contrast Venography

Contrast venography has beenperformed for more than 50 yearss andwas previously considered the techniqueof choice in the detection of DVT, due toits high sensitivity and specificity for thepresence of clots in the lowerextremities.’2 It is the only availablemethod that provides information on allof the vessels in the venous system of thelower extremities, offering superiorresolution when compared with otherimaging modalities, Because thediagnosis of DVT is dependent on thedetection of thrombi in these vascularareas, the diagnostic accuracy of contrastvenography in this situation is high(9670). This method is, however, lessaccurate in the larger pelvic vesselsbecause of contrast dilutions It isimportant to note that while thecomplication rat e associated withcontrast administration may be reducedwith the use of Iow-osrnolar contrastmedia, venography is nevertheless aninvasive procedure.’2 As such, it is ofienassociated with contrast-induced sideeffects such as pain, allergic reactions,nausea, and vomiting.’~ Furthermore,contrast venography yields technicallyunsatisfactory findings in 10°/0to 15°/0 ofstudies. 14 This modality also has limitedfeasibility in severely ill patients andcannot be performed in as many as 10“/Oof patients such as those with poorvenous access, a histo~ of allergicreactions to contrast media, localinfection of the leg, or renalinsufficiency.’

Anatomical changes related to thepresence of chronic thrombi. can make asubsequent diagnosis of acute thrombosisdifficult. At present, contrast venographyis recommended only as an alternativeprocedure and its use should be limitedto patients in whom there is a highclinical suspicion of DVT, or in cases inwhich ultrasound is unavailable or maybe technically inadequate. 14

Impedance Plethysmography

Impedance plethysmography(lPG) is a non-invasive procedure thattakes advantage of the normalphysiologic phenomenon of the venousvolume change in the legs that occursduring respiration. In the presence ofthrombi, the venous volume decreasescorrespondingly. IPG can detect thrombithat produce obstruction in the thigh, butit is unable to detect most thrombi in thecalf. Furthermore, it is unable todifferentiate between DVT and non-thrombotic causes of venous outflowobstruction. Tensing of the muscles, thepresence of extravascular masses, orelevated venous pressure due to thepresence of congestive heart failure canlead to false-positive results. Thepresence of diminished afferent arterialflow to the limbs in patients withperipheral arterial diseases also results indiminished efferent venous flow and canreduce the specificity of the test.7’14

Ultrasound

Compression ultrasound (CUS),which works on the principle of thefailure of the vein to collapse undergentle external pressure, is regarded asthe non-invasive technique of choice fordetecting DVT in symptomatic patients.’5CUS is preferred for initial investigations

4

in the diagnosis of acute DVT g above theknee and is more commonly used thanIPG techniques in these cases. lb CUS is amore comfortable procedure for thepatient and it offers cost advantageswhen compared with contrastvenography. 14 The reported sensitivityand specificity for common femoral veinDVT is in the range of 83% to 100%.Likewise, its sensitivity and specificityfor DVT iri the popliteal fossa is between86% and 100Y0, respectively.7 Resultscompiled from a number of investigationsindicate an overall sensitivity of 95°/0 anda specificity of 98°/0 in the detection ofDVT.2 Although CUS is a quick andreadily available diagnostic test, it ismore difficult to perform in the veins ofthe calf because of the smaller size of thevessels, the slower blood flow andincreased anatomical variance in thelower leg.g Its usefulness in patients withrecurrent thrombosis is limited sincechanges from acute and chronic DVTmay overlap. 17Furthermore, CUS is notreliable in the management ofasymptomatic post-operative patientswho are at high risk for DVT. ‘5’18This ispossibly due to the small size ofpostoperative thrombi as well as thesmall amounts of venous andperivascular tissue inflammatoryinfiltrates in the early and occult stages ofDVT.

Duplex ultrasonography usesreal-time ultrasound techniques and issupplemented by Doppler flow, whichallows the direct imaging of the venouscirculation with simultaneous blood flowinformation, Studies using duplexultrasound techniques for evaluation ofproximal DVT reported high sensitivityand specificity, but its use in detection ofthrombus below the knee is associatedwith much lower sensitivity due to poorvisualization of the calf veins. 19

The accuracy of ultrasoundprocedures employed in the detection ofDVT is dependent on the training andexperience of the operator.

Magnetic Resonance Imaging

Magnetic resonance imaging(MRI) offers high sensitivity andspecificity in the detection of DVT. MRIhas demonstrated an advantage in theimaging of iliac and calf veins. While itshigh cost and limited availabilitydiscouraged its routine use in thediagnosis of DVT; it was later discoveredthat ml is helpful in cases that requirefine anatomical detail to providedecisive information relevant to patientmanagement, 19This form of technologypresents the potential to further theavailability of accurate diagnosis ofDVT, 17

NUCLEAR MEDICINE IMAGINGTECHNIQIJES

Many scintigraphic techniques

currently in use or under investigation foruse in the diagnosis of DVT have thepossible advantage of distinguishingbetween acute, hematologically activethrombi and chronic thrombi, thus aidingin the selection of appropriate treatment.Hull and co-workers20 have shown thatobjective diagnosis followed by selectiveanticoagulation therapy is significantlymore cost-effective than treatment basedsolely on clinical grounds, Aradiopharmaceutical agent that permitsan accurate diagnosis of DVT woulddefinitely decrease the morbidity andmortality associated with this conditionsince appropriate therapy could beoffered accordingly. in economic terms,this would be manifested in savings forboth the patient and hospital, since

5

● hospital stays would be shorter, Adefinitive test should eliminate the needfor additional diagnostic studies, therebyfacilitating the delivery of appropriatetreatment.

The development of any newradiopharmaceutical procedure for use inthe diagnosis of DVT requires theconsideration of a number of factors,including: availability and convenience,the time interval between injection anddiagnostic result, and other issues such asrate of blood clearance, level of softtissue background, specificity of agentfor thrombi, as well as organ uptake.

Radionuclide Venography

The most commonly performedscintigraphic techniques for DVT to dateare based on non-specific radionuclidevenography. Radionuclide venography of

*

the lower extremity performed with Tc-99m macroaggregated albumin can beused in conjunction with a perfusion lungscan. This technique produces images ofthe course of deep veins and collateralroutes. It also may identify the clot itself.

False-positive results can occur in areasof blood pooling, especially in regions ofdilated, incompetent, or perforatedveins. 21Circulation within the deep veinsalso may be visualized with Tc-99m red-blood-cell venography.

The use of radionuclidevenography is hindered by its poorresolution and its inability to identify thethrombus. Since it only identifies the siteof occlusion, it is therefore unable todistinguish between extraluminal andintraluminal causes of thrombosis ordistinguish active (or acute) thrombifrom remitting (or chronic) thrombi. Theresults are variable distal to the poplitealveins and hence, the diagnosis of thrombiin the lower leg is not reliable.

In addition, this imaging techniquecan be labor intensive, requiring the effortsof as many as three staff members inorder to carry out a study. Thediagnostic accuracy of radionuclidevenography is limited and it is rarely usedclinically at present.8 However, Tc-99mred-blood-cell venography may beconsidered when ultrasound and contrastvenography procedures are technicallydificult to perform. It may be ofparticular clinical value when the resultsare conclusive (e.g., definitely positive ornegative). ”

Platelet Scintigraphy

Platelet scintigraphy, which wasinitially introduced to diagnose DVT aswell as directly monitor the efficacy oftherapy, is a labor-intensive technique, Itrequires a radiolabeling procedure thattakes at least two hours during which theplatelets are separated from plasma andother cells. The technique requires asmany as 48 hours to 72 hours foradequate platelet deposition and bloodclearance. Both In- 111 oxine and Tc-99m have disadvantages. In-111 oxine,commonly used for cell labeling, has thedisadvantage of untoward radiationdosimet~, while the preferable Tc-99mhas the disadvantage of a shorter half-life.

It is important to note that asignificant drop in sensitivity andspecificity for thrombi more than one dayold, due to poor uptake characteristic ofolder thrombi, has been reported with theuse of this imaging technique.22 Thrombimust be fresh in order to be successfullyimaged using radiolabeled platelets.23Anticoagulant therapy also may interferewith incorporation of labeled plateletsinto thrombi24 potentially reducing thismodality’s sensitivity.

6

Fibrinogen Uptake and FibrinogenScintigraphy

The use of I-125 fibrinogen hasbeen well documented as a usefulscreening agent for patients at high riskfor developing DVT,25’26Since this agentis no longer available commercially, thisnon-imaging test is rarely used. Anumber of radiotracers with emissionsuseful for imaging have been investigatedas radiolabels on fibrinogen. Fibrinogen,similar to radiolabeled platelets, isincorporated only onto freshly formingthrombi and is relatively insensitive topre-existing thrombi,27 Prompt diagnosisis hindered by prolonged bloodcirculation of fibrinogen and images aregenerally not diagnostic if obtained lessthan 24 hours post injection. Furtherexploration of labeled fibrinogen as animaging agent has been limited due toconcerns regarding its human originbecause of the inherent risk of viraltransmission as well as interference byanticoagulant drugs with incorporation oflabeled fibrinogen into fresh thrombi.22

lrnrnunoscintigraphy

Monoclinal Antibodies. Variousmonoclinal antibodies and theirfragments which target platelets or fibrinare under clinical investigation for use inthe diagnosis of DVT, Monoclinalant ibodies have several advantages overother scintigraphic techniques bytargeting essential components ofthrombi with high specificity and affinity.Studies have shown that these proteinsdemonstrate lower absolute uptake tothrombi than radiolabeled fibrinogen.However, it is possible to obtainscintigraphic images sooner due to morerapid blood clearances of the protein.This makes it possible to investigate

these potential agents using moreadvantageous radiolabels such as Tc-99m.

Both anti-platelet and anti-fibrinantibodies have been investigated.2zZ’28-33Anti-fibrin antibodies show the greatestclinical utility in detecting active venousthrombosis.2g’34 Monoclinal antibodiesdirected against platelets may suffer fromprolonged body background radioactivityand slow blood clearance as they alsolabel circulating platelets. In this case,aged thrombi may not be visualizedbecause of potentially inadequatedeposits of new platelets. Monoclinalantibodies demonstrating high specificityfor deposits of fibrin may have theadvantage of localizing in maturethrombi, which contain high deposits offibrin. Slow blood clearance hasprevented early imaging with monoclinalantibodies. Delayed imaging is requiredseveral hours following injection in orderto obtain diagnostic images even with

smaller Fab and Fab’ fragment s.2g’q0There is also concern regarding potentialhuman a;ti-mouse antibody (HAMA)response. The development ofantibodies derived from humanrecombinant DNA technology could beof value in decreasing immunogenecity.3fi

Numerous antifibrin monoclinalantibodies directed against variousantigenic receptors on the fibrin moleculeare currently under investigation, Anideal radiolabeled monoclinal antibodywould localize with high affinity toreceptors on fibrin deposits of both freshand aged thrombi with rapid andcomplete blood clearance of thecirculating radiolabel.

Monoclinal antibodies that targetantigens on the fibrin molecule includethose reacting with the amino group onthe alpha chain,~7 on the D domain of on-

7

●cross-linked fibrin2g-3g and on cross-linked DD dimer regions.2g The rationalefor developing an antibody with highafinity for the DD domain of cross-linked fibrin is based on a unique epitopefound only in cross-linked fibrin. Thisepitope is abundant throughout thrombiand readily accessible to blood-borneradiopharmaceuticals. It is not readilylost upon initial fibrin degradation, thusmaking older thrombi a potential targetas well, There is the possibility that someof these monoclinal antibodies may havecross reactivity and may bind to cross-

linked fibrin degradation products, butthese proteins would not have reactionswith fibrinogen. Cross-1 inked fibrin itselfis the backbone of the formation of bothvenous and arterial thrombi and hasnumerous antigenic sites availablecompared with its parent fibrinogenmolecule.3g

e

The glycoprotein IIb/lllacomplex (GP llb/llIa) plays an integralrole in thrombus formation by bindingfibrinogen to facilitate plateletaggregation, s~ This GP I1b/llla receptoris expressed only on activated platelets.Therefore, proteins such as monoclinalantibodies that localize or bind to thesereceptors selectively bind to plateletsintimately involved in thethromboembolic event making this anexcellent target for the development ofthrombus-imaging radiopharmaceuticalagents.

Several monoclinal antibodieswith activity at the GP I1b/IIIa receptorare being investigated as potentialthrombus imaging agents. At the sametime, there are other anti-plateletantibodies developed which target otherreceptor sites on activated platelets thatare undergoing clinical trials to determinetheir potential as thrombus imagingagents,22

The advantage of antifibrin andantiplatelet monoclinal antibodies usedin the detection of intravascular thrombiis their ability to recognize specificmolecular sites exposed during variousstages of thrombus formation and Iysis.These antibodies can be digestedenzymatically into fragments to enhanceclearance from the blood pool and topromote more rapid excretion into theurine for prompt scintigraphlc detectionand diagnosis. Repeat imaging procedureswith these proteins would be difficult dueto the potential of HAMA responses,which increases significantly for anysecond administration.

Synthetic Pcpti[les. A promisingalternative to antibodies is the use ofsmall peptides with similar afinities,namely those with platelet affmity.4°These small peptides would be moreadvantageous than complex, highmolecular weight antibodies or theirfragments. Peptides can be producedsynthetically which mimic the “backbone”sequences containing the active targetingportion of the protein, as well as achelating agent for the attachment ofradionuclides. These should be easier andless expensive to produce than morecomplex antibodies.41 In addition, theyare not antigenic and there is no dangerof viral contamination. They also havemore rapid blood clearance thanantibodies.22 These peptides can besynthesized to produce sequences similarto the active sites derived frommonoclinal antibodies that have a highatinity for thrornbi, . .

Earlier peptides have loweraffinity for thrombi and rapid renalclearance resulting in low absoluteaccumulation .40 However, smallerpeptides, consisting of less than 20 aminoacids, which contain a sequence of

8

arginine-glycine-aspartate (RGD), havebeen found to bind to the GP lIb/IIIareceptor, These smaller peptides wereobserved to effectively compete withendogenous fibrinogen for receptorbinding sites.42 In a study involving asmall group of patients, a Tc-99m labeledpeptide was shown to be usefil inimaging peripheral venous thrombi withinone to three hours post injection.43 Thisagent which is designed to localize to theGP Ilb/IIIa receptor on the surface ofactivated platelets is now available as aradiopharmaceutical kit for labeling withTc-99m. This product Tc99rn Apcitide,marketed as AcuTectTM, is presentlyindicated for the detection andlocalization of acute venous thrombosisin the lower extremities. Anotherpeptide, known presently as P748, whichalso has a high affinity for the GP Ilb/llIareceptor, has been shown to providegood image definition of femoral veinand lung thrombi in animals.44 This agentis being investigated for its potential inthe detection of pulmonary emboli.

Disintegrins

Disintegrins (also known asinhibitors of integrins) are small proteinsthat have been isolated from the venomof the American viper snake. Thesedisintegrins bind to the GP IIb/IIIareceptor. It was discovered that theseproteins have native RGD peptidesequences in a highly coiled mannercontaining multiple disulfide cross-Iinking bonds. Among a number ofdisintegrins investigated, iodine-123radiolabeled bitistatin showed the bestlocalizing characteristics in thrombusimaging with good absolute binding tothrombi and rapid blood poolclearance.4574L

Recombinant Tissue PlasminogenActivator

Clinical studies performed withradiolabeled recombinant DNA tissueplasminogen activating factor (rt-PA)demonstrated reliable detection ofproximal and distal vein thrombosis inboth fresh and aged thrombi, and wasunaffected by heparin therapy .47 Tissueplasminogen activating factor, a naturallyoccurring serine protease enzyme withtwo functional sites that allows it to binddirectly to fibrin and induce increases inplasminogen catalytic activity.48This ftbrin-specific thrombolytic agent presentsminimal adverse and immunogenicreactions typical of proteins derived usingrecombinmt DNA techniques.

SUMMARY

A rational approach toward anobjective diagnosis of DVT requires amethod that is both sensitive and specificfor all possible locations of thrombi,including the lower limb. There isobvious interest in the development of areliable diagnostic test for this relativelycommon and life threatening clinicalproblem.

An ideal diagnostic modalityshould be readily available, relativelyinexpensive, sensitive as well as specificfor DVT, yield reproducible results, andbe well tolerated by patients. Currentclinical methods for diagnosing DVT arenotoriously diticult or unreliable.Venography with contrast media isinvasive and presents risks for patientswith allergies, is associated with patientdiscomfort and may even possibly induceDVT. Ultrasound techniques, althoughgenerally accepted, have low rates ofdetection, particularly when imaging belowthe knee and is to a certain extent operator

9

● dependent. Radiolabeled platelets are notuseful in the diagnosis of older thrombi andcomplications may occur with the use ofproducts of human origin, such asfibrinogen, which is derived horn pooledplasma.

It seems that unique antigenicmarkers present only on components ofthrombi may offer opportunities for thedevelopment of novel radiopharmaceuticals.The GP TIb~a wmplex on activatdplatelets is one example of such a target,

Immunoscintigraphy techniqueswith monoclinal antibodies, or theirfragments have shown promise asthrombus-imaging radiopharmaceuticals.However, these radiotracers may sufferfrom immunogenicity problems such asHAMA. The amino acids which comprisethe biologically active portion of themonoclinal antibodies could besequenced and replicated synthetically.

*

These synthetic peptides, and those with.native sequences that have a high affinityfor thrombi, show promise in thedetection of DVT,

Two independent strategies havebeen explored in the development ofthrombi-localizing radiopharmaceuticals.The first is that agents selectivelytargeting platelets should have a higheraffinity for arterial thrombi; and second,agents that preferentially localize in fibrinshould exhibit stronger bindingcharacteristics to older venous thrombi.~1Regardless, all of these agents shouldlocalize in actively forming thrombi,thereby permitting scintigraphic imaging.

A clinically useful radio-pharmaceutical capable of providing anaccurate and timely diagnosis of DVT isneeded to combat a common conditionthat affects aassociated with

●mortality.

large population and issignificant morbidity and

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QUESTIONS

1. Thrombosis can be — .life threatening

;: life savingc. both a and bd. none of the above

2. Thrombosis refers to .stroke

:: the formation of a clotc. occlusion of an organd, hemorrhage

3. Mature thrombus formationswould presently be better imaged withmonoclinal antibodies raised against

fibrin:: fibrinogenc. plateletsd. subendothelial collagen

4. Synthetic peptides offer which ofthe following advantage(s) overmonoclinal antibodies?a. They are minimally antigenic with

little danger of viralcontamination

b. They have higher absoluteafinities for thrombi

c. They localize in both platelets andfibrin in thrombi

d. All of the above

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●5, Pulmonary embolism usuallyoccurs as a result of

venous thrombosis;: hemorrhagec. sepsisd. trauma

6, Data indicate that more thanpercent of cases of pulmonary embolismarise from thrombi in the lowerextremities.

30

:: 50

c. 70

d, 90

7. All of the following are trueexcrpt, synthetic peptides can beproduced to have

a. the active antigen bindingsequences of a monoclinalantibody

b, a chelating agent or site to whicha radiolabel can be attached

c. high molecular weights for betterbody clearance

d. platelet afinity

8. Which of the following is not aclinical feature of DVT?

pain:: swellingc. edemad. vomiting

9, A false-negative diagnosis ofDVT can result in all of the followingexcept .

14

a. needless exposure toanticoagulant therapy

b, clot propagationc. embolizationd. death

10. Which of the following is not apredisposing factor in thrombusformation?

underproduction of fibrinogen:: sepsisc. obesityd. increasing age

Il. Advantages of developing amonoclinal antibody in the detection of

thrombi include the following, except

a, they can be targeted to specificmolecular sites

b, they can be enzymaticallyreduced to their fragments toenhance clearance fromcirculation

c. they have the potential forHAMA responses

d. longer blood pool activity makesdelayed imaging possible.

12. Fibrinogen has been welldocumented for use as a thrombusdetection agent but it is not usedclinically now because

a. it localizes only on freshly .forming thrombi

b. it has a long blood circulationtime

c. it is a product of human origind, is no longer commercially

available

13. The GP IIb/IIIa complex is apotential receptor for developingthrornbus localizing radiopharmaceuticalsbecause it is present only on

a. fibrin on the thrombusb. “ platelets in the bloodc. fibrinogend. activated platelets

14. Radiolabeled platelet scintigraphyis useful in thrombus imaging .as

a. it is a quick and simple process tolabel the cells

b. it localizes in both fresh andmature thrombi

c. the sensitivity of the test is notaffected by anticoagulants

d. diagnosis can be obtained within24 hours of the test

15. The use of clinical examinationalone may give a false-positive diagnosisof DVT in of patients.

1oOA to 40V0

:: 20V0 to 50V0

c. 30% to 60%

d. 40% to 70V0

16, _ is the only available imagingmethod that provides information aboutall of the vessels in the venous system ofthe lower extremity,

contrast venography:: impedance plethysmographyc. compression ultrasoundd. none the above

17. M advantage of radionuclidevenography is its .

superb resolution:: consistent results

c. reduced labor requirementsd. none of the above

18. When imaging DVT, for which ofthe following techniques is there aconcern due to a significant drop insensitivity and specificity for thrombigreater than one day old?

platelet scintigraphy:: impedance plethysmographyc. compression ultrasoundd. magnetic resonance imaging

19. Which of the following offersseveral advantages over other imagingtechniques by offering targeting withhigh specificity for platelets or fibrin?

a, contrast venographyb. platelet scintigraphyc. fibrinogen scintigraphyd, immunoscintigraphy

20. Which of the following has thegreatest clinical utility in detecting activevenous thrombosis?

anti-platelet antibodies:. anti-fibrin antibodiesc. human anti-mouse antibodiesd. none of the above

21. are small proteins isolatedfrom the venom of the viper snake.

integrins:: disintegrinsc. glycoproteins

d. early peptides

●

15

● 22. Which of the following ispreferred for initial investigations in thediagnosis of acute DVT above the knee?

contrast venography:: radionuclide venography

compression ultrasound:. IPG

23. Which is the most common sitefor thrombosis?

the lungs:: the heartc. the braind. the lower extremities

24. Which of the following disorderscan mimic DVT?

ruptured Baker’s cyst:: muscle strain

cellulitis:: all of the above

25< All of the following arepredisposing factors associated withthrombus formation except one. Selectthe inappropriate factor.

excess of erythrocytes:: over production of fibrinogenc. cardiac and vascular disordersd. cholecystitis

16

●

u

o