Radioiodinated 1 -(diethylaminopropyl)-4-phenylpiperazine: A potential brain imaging agent

0047-0740/85 53.00 + 0.00 Copyright 0 1985 Pergamon Press Ltd

Int. J. Nucl, Med. BioI. Vol. 12, No.5, pp. 397-400, 1985 Printed in Great Britain. All rights reserved

Radioiodinated 1-(Diethylaminopropyl)-4Phenylpiperazine: A Potential Brain Imaging Agent R. N. HANSON, L. A. FRANKE and N. WEBB Section of Medicinal Chemistry, College of Pharmacy, Northeastern University and Department of Radiology, Children's Hospital Corporation, Boston, MA 021 IS, U.S.A. (Received 18 March 1985) :ne.prepar~ti~n orradioiodinated 1-(diethylaminopropyl)-4-phenylpiperazine, an analog ofHIPDM,and us tissue distribution in rats are described. The precursor undergoes facile electrophilic radioiodination with radionuclides of iodine at the no-carrier-added level to give isolated yields in the 69-85% range. Biodistribution studies indicate that the radiochemical iswell extracted bythe brain (1.68-1.79%10) and activity is substantially retained for 4 h (> 67%). Brain-to-blood ratios during the0.25-4.0 h period were 15-23: 1. The brain uptake and retention as well as the high brain-to-blood ratios suggest the potential use of this agent for regional cerebral blood flow imaging.

Introduction

amines as potential brain imaging agents. Based upon The increased interest in the application of single- the successful utilization of the 4-phenylpiperazine photon emissioncomputed tomography (SPECT) for moiety in the preparation of radioiodinated adrenal the external quantitation of local cerebral blood flow and myocardial agents.(IO,II) we wished to determine has stimulated the developmentof several new radio- whether its application could be extended to include pharmaceuticals. Of these the most promising are 1231 the radioiodination of diamines similar in structure to labeled N-isopropyl-p-iodoamphetamine (IMP)(I,2) HIPDM . This report describes the synthesis of the and N,N,N'-trimethyl-N'-(2-hydroxy-3-methyl-5- diamine, I-diethylaminopropyl-4-phenylpiperazine (HIPDM)(3.4) (DAPP), the preparation of the radioiodinated deriviodobenzyl)- 1, - 3-propanediamine (Fig. 1). The usefulness of IMP as an indicator of ative, P2SI]DAPP, and the evaluation of [llSl]DAPP as cerebral perfusion has been demonstrated clin- a potential brain imaging agent. ically.(~I) A comparison of HIPDM suggests that although its initial uptake in the brain is lower, it may Materials and Methods be superior for studies performed during rapid changes in blood flow.(9) Although these two com- Synthesis of I-diethylaminopropyl-4-phenylpiperazine pounds have proved quite successful for brain imag- (DAPP) (Fig. 2) ing, problems related to their specific activity and To a solution comprised of 9.7 g (60mmol) of high lung uptake have stimulated the search for 4-phenylpiperazine in 50mL benzene were added better second generation agents. 3.8 g (20mmol) of diethylaminopropyl chloride As part of our program to develop organ selective hydrochloride. The reaction mixture was heated radiopharmaceuticals, we have undertaken an exam- at reflux with stirring for 48 h, then cooled to ination of radioiodinated aralkylamines and poly- ambient temperature and filtered to remove the

[123

Fig. 1. 397

n HIPOM

398

R. N.

HANSON et al.

12SI_ DAPP

Fig. 2.

4-phenylpiperazine hydrochloride (6.7 g, 33.5 mrnol) . The filtrate was evaporated to dryness to give a viscous oil which was dissolved in chloroform and purified by column chromatography on neutral alumina using methanol-chloroform (5 :95) as eluent. The separation gave 1.1 g (20% yield) of the pure product and a substantial quantity contaminated with phenylpiperazine. Repurification of the fractions containing product using flash chromatography on silica gel with methanol--ehloroform (8:92) gave an additional 0.55 g of pure product (10%). The oils were dissolved in ethanol, combined, and acidified by the addition of gaseous HCI. The dihydrochloride of l-diethylaminopropyl-4-phenylpiperazine that precipitated was collected by filtration, rinsed with dry ether, and stored in a desiccator. The product melted at 249-253°C (with decomposition) and had spectroscopic and elemental analyses consistent with the proposed structure.

PXS ODS column using CHpH-e6H 6-IN HCI (50: 50: l) as eluent. The solvent was removed by evaporation and the radiochemical was reconstituted with ethanol-Q .9% saline (I :9). Radioiodination using NalllI (265 jlCi) resulted in a yield of 225 jlCi (85%) of [I31I]DAPP. Tissue distribution studies

Outbred Sprague-Dawley rats of either sex were injected under light ether anesthesia with 0.1 mL of a solution containing the radiochemical [12SI)I-diethyl_ aminopropyl-4-phenylpiperazine (lOp Ci). Groups of rats (3 per group) were sacrificed by ether asphyxiation at 15,60, 120 and 240 min after injection. The tissues of interest were excised, blotted free of blood weighed, and counted in a NaI(TI) welI }' scintillatio~ counter. The total activity in the brain [% injected dose (ID)] as well as the concentration of radioactivity in the tissue (% ID-kg/g) were determined. In a second experiment three groups of rats (5 rats Radioiodination of DAPP per group) were injected under light anesthesia with To a conical reaction vessel fitted with a Teflon 0.1 mL of a solution containing the radiochemical lined cap were added sequentially 25 p L of 0.0I M (lOp Ci) plus varying concentrations of carrier DAPP Na2HPO. solution containing 0.2 mg DAPP, 25 P L (I, 10 or 100pg/O.l mL). The groups of rats were of 0.1 N NaOH solution containing 3.8 mCi Na 12sl, sacrificed by ether asphyxiation at 60 min post adSO pL of 1 M Na2HPO. buffer, pH 7.5, and 10pL of ministration, and the tissues were analyzed as preaqueous chloramine-T solution (10 mg/ml.). The ves- viously described. sel was sealed and agitated for 30 min at ambient temperature in an ultrasonic water bath. The reaction Results and Discussion was terminated by the add ition of lOp L of an The potential of radiolabeled aralkylamines and aqueous solution of Na2S20S (20 mg/mL). The contents of the reaction vessel were then injected onto the diamines as brain imaging agents has been demonHPLC column (LiChrosorb Si-60) and eluted over strated by the development of ['23I]IMP and 120min with a linear gradient of 99% ethanol-I% [123I)HIPDM. The present study was undertaken in ammonium hydroxide (Solvent Aj-hexane (Solvent an effort to overcome or avoid some problems reB) beginning at 10:90 and proceeding to 99: I. A lating to these agents, for example, the lack of single major radioactive component containing reproducibility in labeling IMP via radio isotopic 2.75 mCi (72%) was collected at 10-12 min after exchange, the low specific activity, and the high lung injection. The analysis in two systems gave a single uptake. The l-substituted-s-phenylpiperazines concomponent with an ~ slightly greater than DAPP. stitute a class of compounds for which a variety of Radioiodination with Na l231 (8.7 mCi) gave pharmacologic actions, including interactions with 4.6 mCi (53%) of pure [123I)DAPP plus 1.3 mCi (16%) serotonergic and adrenergic binding sites, has been of [123I)DAPP containing a small amount of an identified . The preparation of the substituted phenylunidentified impurity. The product in this case piperazines is accessible via classical synthetic methwas purified by chromatography on a Whatman ods and readily available reagents. Also the phenyl

DAPP: a potential brain imaging agent

399

Table I. Tissue distribution of radioactivity in rats following intravenous administration of (l1lI]I.diethylaminopropyl-4-phenylp iperazine Tissue Brain Time (min) IS

Total

% ID

1.68 (1.S3-1.7S)

60

1.79 (1.65--1.91)

120

1.S7

240

(1.43-1.79) 1.22 (1.13-1.32)

Adrenals

% ID-kg!g 0.17 (0.13--{).21) 0.19 (0.14--{).24) 0.27 (0.I7--{).38) 0.23

0.69 (0.6S--{).73) 0.90 (0.60-1.26) 1.96 (0.86-3.00) 1.52 (1.14-1.76)

(0.1 S--{),32)

Liver 0.33 (0.27--{).39) 0.37 (0.32--{).42) 0.42 (0,3!Hl.44) 0.52

(0.34--{).64)

Lungs

Muscle

Blood

Thyroid

0.048 (0.30--{).82) 0.034 (0.24--{).38) 0.033 (0.02S--{).047) 0.028 (0.020-0.035)

0.011 (0.009--0.0II) 0.009 (0.007--{).0 II) 0.012 (0.010--{).016) 0.011 (0.009--{).012)

0.22 (0.1!Hl.24) 0.26 (0.23--{).29) 0.26 (0.22-0.28) 0,32 (0.28-0.34)

(% lD·kg/g)

4.93 (4.77-5.14) 4.07 (2.21-5.73) 2.53

(2.14-2.71) 2.19 (2.03-2 .34)

% injected dose x mass (kg)/g tissue. Mean (range) for N ~ 3 rats.

ring is capable of undergoing facile electrophilic substitution reactions and, therefore, neither isotopic exchange nor the introduction of activating groups is necessary to provide the radioiodinated product. The synthesis of 1-(3-diethylaminopropyl)-4phenylpiperazine (DAPP) was achieved in one step with an isolated yield of 30% which, although not outstanding, was better than the overall yield reported by Pollard et a/.(12) who obtained the same product in 2 steps via 1-(3-chloropropyl)-4phenylpiperazine. The DAPP readily underwent electrophilic iodination with the three radioisotopes of iodine (NCA) at pH 7.4 to give the desired products in good (69-85%) isolated yields using HPLC. The radiochemicals were 98% + pure, having been separated from starting material, and were identical to the DAPP as demonstrated by TLC and HPLC. The concentrations of radioactivity in the various tissues of the rat at IS, 60, 120 and 240 min after injection are shown in Table I. As the data indicate, the radiochemical was rapidly extracted by the brain, 1.68% ID at IS min. The total activity in the brain remained relatively constant during the first hour before declining slightly by 2 h. In addition to the brain several other tissues demonstrated high concentrations of radioactivity. The lungs, which have been observed to accumulate high doses of other radiolabled aralkylamines, had 4.93% ID-kg/g at IS min; however, this decreased steadily over the course of the study, reaching a level of 2.19%ID-kg/g at 240 min, 43% of the IS min value. The liver values increased during the time period evaluated, from 0.33 to 0.52 %ID-kg/g which probably reflected the metabolic and excretory activity of the organ . The level of activity observed in the adrenals also increased over the time period from 0.69 %ID-kg/g at 15 min to 1.96

and 1.52 at 120 and 240 min, respectively. The concentrations of activity in nontarget tissues, such as muscle, bone and fat were much lower and tended to decline from their IS min values. Activity in the thyroid, an indication of in vivo deiodination, was low, ranging from a value of 0.22 to 0.32 %ID at 240 min. Blood levels remained virtually constant throughout the entire time period of the study. Overall, the tissue distribution of [I25I1DAPP strongly resembles that reported for both [I25I]HIPDM and its analogs, and [I23I)IMP, with regard to the amount of activity present in the various organs and its elimination or retention . For all three radiochemicals the difference between the peak brain level and the last value (determined 2-4 h post administration) was less than 30% indicating that the retention of [I2.5I)DAPP was comparable to that of the other two agents. As had been previously noted, the labeled aralkylamines, when given at low doses (high specific activity), are extensively localized in the lungs and liver. [I25l)DAPP behaves in a similar manner, demonstrating high lung uptake at 15min that gradually increases. When one considers the lipophilic nature of the compound and the presence of amine groups, the observed liver activity can probably be attributed to hepatic metabolism and clearance. Increasing doses of carrier diamine were added to determine whether there existed a specific binding compartment, as is observed with receptor binding radiopharmaceuticals, or a shift in tissue distribution of activity which had been demonstrated with HIPDM. The results are shown in Table 2. The effect of using increasing doses of carrier diamine upon the uptake of radiochemical in the brain and lungs was not so striking as that reported for HIPDM. There were slight increases in brain uptake and brain-to-

Table 2. Effectof carrier on tissuedistribution of radioactivity in rats at 1 h after Lv. administration of radiochemical Tissue Brain

I jig 10jig 100jig

% ID 1.39± 0.08 1.42 ± 0.11 1.88 ± 0.24

Mean ± SO for N = 5 rats.

% lD·k8!g 0.16 ± 0.01 0.16 ± 0.01

0.19 ± 0.03

Liver

Lungs

0,36 ± 0.03 0.36 ± 0.04 0.33 ± 0.06

3.09 ± 0.37 2.79 ± 0.40 3.29 ± 0.44

Muscle

Blood

3.09 ± 0.37 0.036 ± 0.004 0.037 ± 0.002

0.012 ± 0.001 0.012 ± 0.001 0.011 ± 0.001

(% lD.kg/g)

400

R. N. HANSON et al.

blood ratios at the highest doses employed (0.5 mg/kg), but the other tissue levels, including lung uptake, were essentially unchanged. Although the maximum dose used in this study was substantially lower than most of those used by Kung et al., (0.1-25 mgfkg) to enhance brain localization and decrease lung uptake for their agent,ll) similar results might have been observed with this agent if higher doses had been used . However, because of the pronounced activity associated with l-substituted-sphenylpiperazine it was felt to be prudent to use subpharmacologic doses of the drug in this study. In summary, we have described the preparation and evaluation of a new potential brain perfusion imaging agent. Because the radioiodine can be introduced using classical electrophilic conditions, isolation of the product using HPLC gives a pure radiopharmaceutical having high specific activity. The tissue distribution of the new agent in rats is comparable to that of IMP and HIPDM in the same species. Further studies will involve the evaluation of the agent in larger animals and the examination of a series of analogs with potentially greater uptake in and selectivity for the brain. Acknowledgements-This work has been supported in part by a research and scholarship development grant from

Northeastern University, Public Health Service Grant lROI.HL·24666, Biomedical Research Support Grant S07 RR05830, and a U.S. Department of Energy Contract

DE-AC02-76EV0411 S. The authors acknowledge the assislance of A. Camel, T. Manning and D. lohnson with the animal studies and R. Taube and R. Aveline with the editing and typing of the manuscript.

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