Hepatobiliary Handling of Iodine-125-Tyr@Octreotide and Indium-i 11-DTPA-D-Phe1Octreotide by Isolated Perfused Rat Liver Marion de Jong, Willem H. Bakker, Wout A.P. Breeman, Marcel E. van der Pluijm, Peter P.M. Kooij, Theo J. Visser, Roelof Docter and Eric P. Krenning Departments ofNuclear Medicine and Internal Medicine HI, University Hospital Dijkzigt and Erasmus University Medical Schoo4 Rotterdam, The Netherlands
RadiOlabeled bioa@ve peptides may show receptor-mediated binding to tumors, making them suitabis for ScintigraphiCimag
ing.Theliverisan important organforpeptideclearance.Togan insightintothe uptake and intracellularprocessingof somatosta
(1@yr@-octreotide) has been developed in which a phenylala nine has been replaced by tyrosine, allowing radioiodina tion of the molecule (Fig. 1C). This compound, radiola beled with 1251or 1@I, has been used successfully for in
vitro somatostatin receptor studies (4—7)and tumor scin tin analOgS, we compared the hepatobiliaiy handling of ‘@l-Tyr@ tigraphy in animals (6,8) and humans (1,9,10). Another octreotide and 111In-DTPA-D-Phe1-octreotide, which are suc radioactive analog of somatostatin is “In-DTPA-D-Phe' cessfullyused to image somatostalin receptor-positivetumors in octreotide (Fig. 1D), which is also used for in vivo scintig vivo in isolated recirculating perfused rat livers. Sixty minutes raphy. The last compound lacks some drawbacks of 1@I@ followingadministration of the radiolabeledpeptides,perfusion Tyr@-octreotide(7,8). mediumand biliaryradioactivity were analyzed.Radiolodinated The liver is an important organ in the degradation of Tyr@-OctreOtIde was rapidlycleared by the liverand 60% of the many circulating peptides. We compared liver handling dose was excreted intact intothe bile after 60 mm. In contrast, and excretion into bile of ‘@I-1'yr@-octreotide and “In 1 1 1ln-DTPA-D-Phe1-octreotide was not cleared by the liver; me diumradioactMtylevels remanedaboutconstantandonly2%of DTPA-D-Phe'-octreotide in isolated recirculatingperfused the dose was foundinthe bile.These resultsare inagreement rat liver. By studying the uptake and intracellularhandling with in vivo findings in rats and humans. We concluded that of octreotide analogs in an isolated rat liver perfusion sys isolated ratkver perfusion is a good system to rapidly gain insight tem, we hope to further extend our understandingof the intothe hepatichandlingof radiopharmaceuticals. pharmacokineticbehavior of these compounds. J NucIMed1993;34:2025-2030 MATERIALS AND METhODS Materials Theradiopharmaceuticals usedwereNA@I(AmershamInter umor receptor-bindingradiolabeledpeptides are inter esting recent developments in nuclear medicine, as they can be used for in vivo scintigraphic imaging of tumors. An example is somatostatin (Fig. 1A), which binds to its re ceptors on tumors of neuro-endocrine origin (1). This native peptide is susceptible to veiy rapid enzy
matic degradation(2), and therefore is not very useful for in vivo application. For that reason, more stable synthetic somatostatin
analogs have been developed.
The octapep
tide octreotide (SMS 201-995 or Sandostatin®, Fig. 1B) fulfils this criterion (3). Large numbers of high-affinity binding sites for native somatostatin and synthetic cc treotide have been detected on most endocrine-active tu mors (4). Since octreotide cannot be radiolabeled easily with a gamma-emitting radionuclide, a synthetic analog Rece@ Jan.28,1993;
[email protected],1993.
national, UK), “mCi3(Mallinckrodt Medical BV, Petten, The
Netherlands),Tyr@-octreotide and DTPA-D-Phe'-octreotide(San doz PharmaAG, Basel,Switzerland),‘3'I-human serumalbumin (Sorin Biomedica, Tronzano, Italy) and bovine serum albumin
(OrganonTeknika,Oss,TheNetherlands).Bovineserumalbumin (BSA)andhumanserumalbumin(HSA)werehighlypurifiedand free of fatty acids. All other reagents were of the highest purity
commerciallyavailable. Radloiabellng Radioiodinationof Tyr@-octreotide with ‘@I was performed with the chloramine-Tmethodas describedby Bakkeret al. (6). Labeling of “In-DTPA-D-Phe'-octreotidewith “Inwas carried
out as previouslydescribed(7). Isolated Perfused Rat Uver System Livers of male Wistar rats (200—250 g) were isolatedand per fused in a recirculating system at 37°C as described by Docter et
al. (11). The magneticallystirredperfusionmediumused in all experimentswas 150ml of Krebs-Ringerbufferwith with Naa
For correspondenceor repdn@cont@t Marionde Jong, UniversityHospital pac@gt,Depi@ of NuciewMed@ne,Dr.Molewaterplaln 40, 3015 GD R@teidwn, (118 mmol/liter), KCI (5 mmol/liter), Mg504 (1.1 mmol/liter),
TheNetherlands.
UverHandlingof SomatostatinAnalogs• de Jongat al.
c@a2 (2.5mmol/liter), KH2PO4 (1.2mmol/liter) andNaHCO3 2025
radioactivity. Radiochemical composition of the different samples
AlSomatostatin
was confirmedby HPLCanalysiswitha Waters600E multisol vent delivery system connected to a p@-&ndapak-C18 reversed phase column (300 x 3.9 mm, particle size 10 Mm).Before HPLC,
I-
I
Ala-Gly-Cys-Lys-Asn-Phe-P/ze-Tip-Lys-T/zr-Phe-Thr-Ser-Cys
bilesampleswerediluted1:10with40%methanolin 154mMof NaC1. Elution was carried out at a flow of 1 ml/min with a linear
gradientof 40%to 80%methanolin 154mMof NaC1in 20 mis. The lattercompositionwas maintainedfor another5 mis. Col lected fractionswere measured by routine scintillationcounting. BI Octreotide
Calcuiatlons Curve-fitting of the two-exponential medium tracer disappear ance curve was done as described previously (11). A peel-off
system of curve-fitting was used (Fig. 3). All data are reported as mean ±s.d. of each parameter obtained in replicate studies (n =
D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr(ol)
4—8). StatisticalevaluationwasperformedusingStudent'st-test.
RESULTS IodIne-I25.1@ctreotIde
C [‘2'I-Tyr'J-octreotide
Figure 2 shows typical time courses of total radioactiv ity, peptide-bound
radioactivity
and liberated
1@I in the
medium and bile after administering‘@I-Tyr@-octreotide to the perfusion medium. Results are expressed as a percent age of the administereddose. After tracer administration, radioactivity rapidly disappears from the perfusion me dium followed by rapid excretion into the bile. Of the administered radioactivity, 27% of peptide-bound radioac
D-Phe-Cys-Tyr-D.Trp-Lys-T/zr-Cys-Thr(ol)
tivity is left in the perfusion medium after 60 min, while
D [‘‘ln-DTPA-D-Phe]-octreotide
about 1% consists of free iodide (Table 1). Free iodide in the bile accounts for only 1%of the administeredradioac tivity; most of the radioactivity
FIGURE1. Somatostatln andanalogswiththesupposedbloac live site printed In Italics. mmol/liter) supplemented with 10 mM of glucose and 1% BSA.
(60% of the total dose) is
excreted in peptide-boundform in the bile. HPLC analysis revealed that the peptide-boundradioactivityin the bile is completely intact 1@I-Tyr@-octreotide. In all experiments, the disappearanceof ‘@I-Tyr@-octreotide could be fitted to the sum of two exponentials. In Figure 3, this two-expo nentialmedium disappearancecurve is shown and the half
The pH of the mediumwas maintainedat 7.43 by gassingwith lives of the fast and slow component were calculated. carbogen (95% CO2 and 5% 02, 400 mI/mis). Liver function was The distributiontime throughthe system was estimated monitoredby its outer appearance, hydrostaticpressure neces by perfusing livers with ‘31I-HSA, a substance that is not saly to maintaina perfusionmediumflowof 40ml/min,bile flow taken up into the hepatocytes. The half-life of the fast andpHof theperfusionmedium.Liverswerepreperfusedfor30 component of the medium ‘311-HSA curve was 0.29 (0.02) mm. The experiment was started by adding 370 kBq of tracer min, whereas the half-life of the slow component was very (10@mole)to the stirredmediumin the centralreservoir.Sub long as almost no radioactivity disappeared from the per sequently,0.5-mImediumsampleswere takenat 1—10 mmand 15, fusion medium. In the case of “@I-Tyr@-octreotide, the 20, 25, 30, 40, 50 and 60 mm from a smaller medium reservoir, withhydrostaticpressuredeterminedby height.Forthedeterini half-life of the fast component was 0.28 (0.04) mm (repre nation of correct curve-fitting, more samples were taken in the senting distribution through the perfusion system) and 36.2 first minutes of some experiments. Bile samples were collected (3.9) min of the slow component (representinguptake and during 10-mm intervals. The samples were stored at —20°C until
analysis. Perfusion Medium and Bile Sample Analysis The chemicalstatusof the radionucidein the perfusionme dium and bile samples was analyzed as a function of time using
SEP-PAKC18chromatography.The perfusionmediumand bile sampleswere appliedto SEP-PAKC18columns,whichhad been activated with 2-propanol (5 ml). Elution of the different fractions
metabolism,
Table 2).
Indium-I I 1-DTPA-D-Phe'-Octreotlde Figure 4 shows typical time courses of total radioactiv ity, peptide-bound radioactivity and nonpeptide-bound breakdown products after administration of “In-DTPA
D-Phe'-octreotide to the perfusion medium. After 60 min, 95% of the administered radioactivity still consists of pep
wasperformedwith5 mlof distilledwaterand5 mlofO.5M acetic tide-bound tracer in the perfusion medium, which is iden acid to removefree ‘@I, and 5 mlof 96%ethanolto elutepeptide tical to intact “In-DTPA-D-Phe'-octreotide as demon bound radioactivity. Fractions were collected and counted for strated by HPLC (not shown) (Table 1). Only a small
2026
The Journalof NudearMedicine• Vol.34 • No. 11 • November1993
TABLE I
A
Original Peptides and Break-Down PrOdUctS
100
PBRNPBRtNPBR(medium)(medium) PBR (bile)
80
1@I-Tyr'-OctreOtIde 26.9(1.9) 1.2(0.0) 60.4 (3.7) 111I@W@@@
94.9 (0.8)* 0.9 (0.1)*
(bile) 1.0(0.1)
2.2(O.2)* 0.2(0.0)@
Phe1-o@ Cl)
0 •PBR — peptide-bound
@
tNPBR = nonpeptide-boundradloa@Mty,e.g., 1@land 111ln-DTPA, in perfusionmedIumand bileafter60 mmof perfusionw@ @Tyr' oc@rec@kte or ‘“In-DTPA-D-Ph&-oc@recAide. Resultsaregivenas mean
20
(s.d.)%dose(n= 4—8). *p < 0.001 versus 1@l-Tyr'-octreotIde.
0
radiopharmaceuticals
0
10 20 30 40 50 60 minUtes
B
r@1ioactivlty.
40
70
have been investigated in vitro (4-7)
and in vivo (1,48—10),little is known about the metabo lism ofthese radioligands, especially in the liver. However, there is much information regardingthe hepatic metabo lism of the native peptides somatostatin-14 and somatosta tin-28. They are degraded through the action of hepatic aminopeptidases and endopeptidases as studied in the per fused rat liver (12,13). The perfused rat liver is very suit
60
,@
@
50
able for investigating several parameters of liver metabo lism, such as the disappearance from the perfusion medium and the appearance of degradation products and biliary
40
excretion. Therefore, in this study we compared the liver handling of ‘@I-Tyr@-octreotideand “In-DTPA-D-Phe1-
30
octreotide in this system. Iodine-125-Tyr@-octreotideis rapidly taken up into the
20
1
10 0 10
20
30
40
50
60
minUtes U)
FiGURE2. (A)Ratliverperfusion:a typicalexampisof disap
0
pearance of total (0) and peptide-bound (•)radio@Mty from the
medkimand appearanceof 125@ (A)Inmediumafteredministration of 1@l-Tyr@-octre@Ide. (B)Typicalexampleof cumulativeexcretion @
by the perfused rat liverof total rathoactMty (•), dMded in peptide bound redioactlvfty(open bar) and (@c@f bar), Intothe bile after
administrationof 1@t-Tyr'-odreotlde. portion (2%) of the administered radioactivity is excreted in the bile. Furthermore,the half-lives of the fast and slow components of the biphasic disappearance from the me dium (which is described as the sum of two exponentials)
are 0.25 (0.08) min (distribution) and unmeasurably long, respectively (Table 2).
0.1
0
10 20 30 40 50 60 minUtes
FIGURE3. Typicalexampleof thecurve-fitting of a 1@l-Tyr@octreotidedisappearancecurve,whichrepresents the sum of two
DlS@USSlON
exponentials,by a two-exponentialmodel. Piot of log %dose/ml
Radiolabeled octreotide analogs bind to the somatostatin receptors on neuro-endocrine tumor cells and are suitable
versus time, with the least-squares regression line on the final straightpartofthe ctnve (sbw component@. Inset logpiotofthe fast component data values were obtainedafter subtractingthe siow componentw@the least-squaresregression @e.
for scintigraphic imaging of these tumors. Although these
LiverHandlingof SomatostatinMalogs • de Jong at al.
2027
TABLE 2 Haif-Uvesof the Fast and Slow Disappearance Components
A
from the Medium of 131l-HS@1@I-Tyr@-Octreotide and @
100
I
80
Fast
com@131IH5p@
com@Slow
(0.02)oo@1@I-Tyr@-octreotide 0.29
4) U)
(0.04)36.2(3.9)111In-DTPA-D-Phe1-0.28
0
(0.08)@toc@eodde*@@jft@ 0.25
@
40 4—8).tp aregivenInmean (s.d.) minutes(n =
< 0.001 versus 1@l-Tyr@-octreotIde.
20 liver, immediately followed by intact excretion into the bile (Fig. 3). Because the half-life of the fast component is the same as that of ‘311-HSA—-which is not transported into the hepatocytes
0@
0
but is passively distributed into the liver inter
10 20 30 40 50 60
stitium—itis predominantly determined by distribution of the tracer through the system and the extracellular liver compartment.
The half-life of the second component
minUtes
is
mainly determined by transport and metabolism in the liver. We recently reported transport and metabolism of thyroid hormones in the perfused rat liver where an uptake and metabolism component could be distinguished clearly in the biphasic disappearancecurve from the medium after the distribution phase (11). In the case of ‘@I-Tyr@-oc treotide, uptalce is immediately followed by excretion into
the bile and therefore not seen as a distinct component. Medium and bile chromatographyshowed that hardly any free radioactive iodide is found, while the majority of the administered dose is already excreted intact into the bile within 60 min. Very little of the administeredradioactivity accumulates
B
3
2 4) Cl)
0
1
in the liver.
An internalized peptide can reach the bile by two path ways: (1) a lysosomal, or indirect pathway or (2) a nonly sosomal,
or direct pathway
(14). In general,
molecules
processed by the first pathway are not excreted intact into the bile. Compounds that utilize the second pathway are generally excreted into the bile as intact molecules and
@
0 10
20
30
40
50
60
minUtes
appear in the bile sooner than those excreted by the lyso FIGURE 4. (A) Rat liver perfusion:a typicalexample of disap somal pathway. Our results show that ‘@I-1@r@-octreotide pearance of total (0) and peptide-bound(•) radioactivIty fromthe is translocated across the hepatocytes into the bile by the mediumandappearanceofnonpeptide-bound breakdown products direct pathway, thereby bypassing the lysosomes, because (A) in medium after adminletration of 11In-OTPA-D-Phe1-octreo the radiopharmaceuticalis excreted intact into the bile and tide. (B)A typical example of cumulative excretion by the perfused because there is no lag time before excretion into the bile.
rat lNerof total radioactivIty(S), dMded m peptide-boundracloac tMty (open bar) and nonpeptide-bound breakdown products (solki
These findings are in accordance with 1@I-T@rr@-octreo bar)intothe bileafter111In-DTPA-D-Phe1-octreotide administration. tide's stability against (hepatic) enzymatic degradation, con
traly to the native peptides, somatostatin-14 and somatosta tin-28 (12,13). This is probably due to the introduction of a D-amino acid at the N-terminal and an amino-alcohol sub stituent at the C-terminalend of the peptide chain (3). Recently, it has been reported that uptake into isolated
rat hepatocytes is a carrier-mediatedprocess which is re lated to the multispecific bile acid transporter for several types of cyclosomatostatins (15). Further transcellular
bile-acid transportto the bile after uptake into the cells has
2028
been elucidated by electron microscopic autoradiography, showing that these substances are excreted rapidly into the bile in a chemically unchanged form (16). We did not investigate possible carrier-mediatedtransport properties of our somatostatin analogs, but intact excretion into the bile after a rapid transcellular transport agrees with our findings in the perfused rat liver. The results of our study are in agreement with rat and
The Journalof NudearMedicine• Vol.34 • No. 11 • November1993
administration, the radioactivity measured above the liver does not increase, but decreases,
4)
due to clearance by the
kidneys rather than the liver (8 10). Likewise, no radioac @
@
600 .@
500
Cr)
400
tivity could be measured scintigraphically
300
I to: ..@;‘
200
0
5
10
15
20
25
mm after injection FIGURE5. RacfioactMty, expressedas mean±s.d. percentage of the 3-mmvalue,measuredaboveROlein humans,i.e.,liver tissue wIthoutmajor bile ducts (after 1@l-Tyr@-octrectIde (•,n =5)
and111ln-DTPA-D-Phe1-octreotide (0, n = 3))andgallbladder (after 1@I-Tyr@-octreotide (A, n = 5)). After injection of 111In-DTPA-D-
Phe1-octreotlde, no bile-related radioactivitywas measured Inmajor
bile-dUCtS or Inthe gallbladder.
above the gall
bladder. Indium-111-DTPA-D-Phe1-octreotideis the preferred analog for in vivo scintigraphy because it has several ad vantages compared to ‘9-1@yr@-octreotide: general avail ability, simple one-step radiolabeling, longer physiological half-life in plasma and a more suitable metabolism. In order to visualize a tumor by receptor binding in vivo, the spe cific activity expressed in counts per unit of area must exceed the local background radiation. Unlike radioiodi nated 1'yr@-octreotide, “In-DTPA-D-Phe'-octreotide is not cleared via the liver and causes no accumulation of radioactivity in the biliaiy and digestive tracts. The latter radiopharmaceutical is more suitable for visualization of tumor-receptor accumulation in the upper abdominal re gion where the small endocrine gastro-entero-pancreatic target tumors are located. This study showed that the results obtained in the per fusion system agreedwith in vivo results and could predict in vivo clearance for both rats and humans. Currentlywe are exploring the use of other radiolabeledpeptides which will be used for imaging endocrine tumors and immunolog ical disorders. As we prefer to use peptides that are not degraded in or cleared by the liver and therefore cause no accumulation
of radioactivity
in the biliary and digestive
tracts, the liver perfusion system can be used to test liver human in vivo investigations with ‘@I-1'yr@-octreotide, metabolism of these peptides rapidly. During in vivo stud which show that disposal occurs predominantlyby rapid ies in rats and humans with ‘@I-1@r@-octreotide and other uptake into the liver and excretion via the bile into the peptides, it is difficult to determine which fraction of intestines (6,9). In Figure5, metabolism in the humanliver plasmaradioactivityis still composed of intact materialand is shown. Data are derived from studies by Bakker et al. where peptide degradation takes place—inthe liver, the (9). Iodine-123-1@yr@-octreotide is rapidly cleared from the intestines or the kidneys. Rat liver perfusion studies may circulation by the liver, immediately followed by hepato provide insight into this matter. Furthermore, when it is biiary excretion; radioactivity in a nonmajor bile duct known where and how a compound is degraded, possible containing part of the liver does not increase above 150% inhibition of early and undesirable degradation can be in of the 3-mis value, but a sharp increase of radioactivity is vestigated to save intact compounds for binding to their seen above the gallbladder during the first 20 mm after receptors on tumors. The rat liver perfusion system can administration. also be used for these studies. Indium-111-DTPA-D-Phe'-octreotide is not taken up by the isolated perfused rat liver because almost no tracer REFERENCES disappears from the perfusion medium. The half-life of the fast component is againdeterminedby distributionthrough 1. KrenningEP, BakkerWH, BreemanWAP,et al. Localizationofendocrine related tunx@rs with radiolodinatedanalogof somatostatin.Lance: 1989;!: the system, whereas the half-life of the slow component is 242-244. very long, indicatingvery slow handling in the liver. Fur 2. PatelYC, Wheatley1. In vivoandin vitroplasmadisappearance and thermore, hardlyany radioactivity is found in the liver and metabolismof somatostatin-28and somatostatin-14in the rat. Endocrinol o@,1983;112:220—225. excretion into the bile is negligible. This may be due to the 3. PleasJ, BauerW, BrinerU, Ctal. Chemistjyandpharmacology of SMS additionof the relatively large andvery hydrophylicDTPA 201-995,a long-actinganalogof somatostatin.Scandi Gastmenterol 1986; groupto the octapeptide molecule, favoringrenalexcretion 21(suppl119):54—64. 4. ReubiiC,HackiWH, Lamberts SWL.Ho@one-pmducinggastrointesfinal of the latter, like the radiolabeledchelate itself (i.e., @‘@‘Fc tumors contain high density of somatostatinreceptors.I Clin Endociinol DTPA), which is excreted exclusively by glomerular filtra Metab 1987;65:1127-1134. tion (17). These findings are in excellent accordance with 5. ReubiJC. Newspecific perfusionradiohgandforone subpopulationof brain somatostatin receptors. Life Sd 1985;36:1829-1836. in vivo studies (8,10). In Figure 5, metabolism in the hu 6. BakkerWH,KrenningEP,BreemanWA,Ctal. Receptorscintigraphy with man liver is shown using data derived from studies by a radioiodinated somatostatinanalog:radiolabeling,purification,biologic Krenning
et al. (10). After
“In-DTPA-D-Phe'-octreotide
UverHandlingof Somatosta@n Analogs• de Jongat al.
activityand in vivo applicationin animals.JNuclMed 1990;31:150!-1509.
2029
7. Bakker WH, Albert R, Bruns C, et aL Indium-111-DTPA-D-Phe'-oc system: albumin does not play a role in cellular transport. Endoa*sology 1990;126:451—459. treotide,a potentialradiopharmaceuticaifor imagingofsoinatostatinrecep tor-positivetumors:synthesis,radiolabelingand invitrovaiidation. Ufe Sd 12. SacksH,TerryLC. Clearanceofimmunoreactive somatostatin byperfused 1991;49:1583—1591.
rat liver. I Clin Invest 1981;67:419—425.
8. BakkerWH, KrenningEP, ReubiJC, et al. In vivo applicationof “In 13. Ru@ere MD, Patel Y. Hepatic metabolism of somatostatin-14 and soma DTPA-D-Phe'-octreotideforthe detectionof somatostatinreceptor-positive tostatin-28:immunochemicalcharacterizationof the metabolicfragments tumors in rats. Life Sd 1991;49:1593—1601. and comparisonof cleavagesites. Endocthzology1985;117:88-96. 9. BakkerWH, KrenningEP, BreemanWA,et aLInvivo use of a radioiodi 14. Coleman R. Biochemistry of bile secretion. Biochem I 1987;244:249-261. natedsomatostatinanalog:dynamics,metabolismandbindingtosomatosta 15. ZieglerK, LineW,Frimmer M.Hepatocellular transport ofcyclosomato tin receptor-positivetumors in man.JNudMed 1@132:11M-1189 statins: evidence for a carrier related to the multispecificbile acid trans 10. Krenning EP, Bakker WH, Koo,j PPM, et aL SomatOstatin receptor scin porter. BiochbnBiophysActa 1991;1061:287-2%. tigraphy with “In-DTPA-D-Phe'.octreotide in man: metabolism, dosime h3r and comparisonwith
‘@I-Tyr'.octreotide.JNudMed
199233:652-658.
11.DocterR, DeJongM, VanderHockHi, KrenningEP,Hennemann 0. Developmentand use ofa mathematicaltwo-poolmodelofdistributionand metabolismof 3,3',5-triiodothyroninein a recirculatingrat liver perfusion
16. Frimmer M, Ziegler K. The transport of bile acids in liver cells. Biochim
Bkç1@ysActa1988;947:75-99.
17. KlopperJF, HauserW,AtkinsIlL,Eckelman WC,Richards P.Evaluation of@Fo-D1PA for measurementofglomerularfiltrationrate. JNud Med 1972;13:107—11O.
EDITORIAL
HepaticHandlingof Radiopharmaceuticals:Is the InVitro ModelUseful? S°@ polypeptide
is a hyothalamic that inhibits the se
cretion of the pituitary growth hor mone. It also inhibits the secretion of prolactin and thyroid-stimulating hormone and has a variety of other inhibitory
effects.
These
divergent
functions of hormones are generally related to the biodistribution of their corresponding receptors. The recep
tors for somatostatin are expressed in the brain, the anteriorlobe of pitu italy gland, acinar and islet cells of
the pancreas, stomach mucosa, intes tinal mucosa and the adrenal gland.
In addition, these receptors are also expressed on tumor cells of neuroen docrine origin, including meningioma, gastrinoma, carcinoid and insulinoma.
Naturally, somatostatin is considered as a reagent useful for in vivo scinti graphic imagingas well as for therapy of such tumors. However, native forms of soma tostatin that are 14 or 28 amino acid peptides (somatostatin-14 and soma tostatin-28, respectively) have very short biological half-lives; therefore, their clinical usefulness is limited. They are metabolized very rapidly through the action of aminopeptidases and endopeptidases principally in the liver. Synthetic somatostatin analogs ReceiverlAug 18 1993;aoneptedAug.18,1993.
Forcorrespondence ormprir@s onrd@t H.Talca
h@N. MD,PhD, MolecularHep@loqy Lalxxstoiy. MGHCancer, @hate@n, MA02129.
2030
were synthesized to increase their sta bility in vivo (1). An octapeptide cc treotide, SMS 201-995, is a somatosta tin analog that possesses D-isomer of phenylalanine (D-Phe) and amino al cohol of threonine (Thr(ol)) at N-ter minal and C-terminal end, respec tively. This analog is resistant to proteolysis and has a long half-life. It has been used for the treatment of growth hormone-producing pituitary adenoma and gastrinoma (Z3). Fur thermore, derivatives of this analog have been labeled with radionucides
patic accumulation was not observed (6). The modification of the N-tenth
nal D-Phe residue with the “In DTPA group appears to have inhibited hepatic clearance. The difference in metabolism and biodistnbution be tween [‘@I-Tyr@]-octreotideand [“In-DTPA-D-Phe'l-octreotide was confirmed by an in vivo system in man (7). Thus, the modifications of pep tides with differentradionudides may change their metabolism and biodistri bution in vivo. The liver andthe kidney are the two major organs for metabolism and to visualize somatostatinreceptorscx pressed in tumors of neuroendocrine clearance but more so the liver be origin. Initially, a phenylalanine of cc cause it is located in the center of the abdominal cavity and by itself causes treotide has been replaced by tyrosine to allow iodination (‘@I or ‘@I-Tyr@a high background. In addition, me tabolized radionucides may be se octreotide), and nuclear imaging of en docrine-related tumors was tested (4). creted into the intestine through the However, highabdominalbackground biliaiy tract, thereby increasing ab was a major drawback. Radioiodi dominal background and interfering nated Tyr@-octreotideswere rapidly with nuclear imaging. As the modifi cations of the peptides with different cleared from the circulation princi radionucides seem to change their pall)T through the liver and secreted metabolism in the liver, the availabil into the biliaiy system. This hepatobil jar), clearance resulted in high hepatic ity of an in vitro system to examine and intestinal accumulation (5). Sub hepatic handling of modified reagents will help us to understand the pharma sequently, a diethylenetriaminepen cokinetics of bioactive reagents and taacetic acid (DTPA) has been conju gated to phenylalanine of octreotide may be useful in predicting their be for ‘DInlabeling ([“In-DTPA-D haviors in vivo. Such a system also will provide information on the uptake Phe']-octreotide). In contrast to radio iodinated 1'yr@-octreotide, “SIn and intracellular processing of re DTPA-D-Phe'-octreotide was cleared agents that are difficult to study in predominantly by the kidneys and he vivo.
TheJoumalof
NuclearMeducine•Vol.34•No. 11•November1993
