title: Impact of clinically tested NEP/ACE inhibitors on tumor uptake of [(111)In-DOTA]MG11-first estimates for clinical translation.
abstract: BACKGROUND
  We have recently shown that treatment of mice with the neutral endopeptidase (NEP) inhibitor phosphoramidon (PA) improves the bioavailability and tumor uptake of biodegradable radiopeptides. For the truncated gastrin radiotracer [(111)In-DOTA]MG11 ([(DOTA)DGlu(10)]gastrin(10-17)), this method led to impressively high tumor-to-kidney ratios. Translation of this concept in the clinic requires the use of certified NEP inhibitors, such as thiorphan (TO) and its orally administered prodrug racecadotril (Race). Besides NEP, angiotensin-converting enzyme (ACE) has also been implicated in the catabolism of gastrin analogs. In the present study, we first compared the effects induced by NEP inhibition (using PA, TO, or Race) and/or by ACE inhibition (using lisinopril, Lis) on the biodistribution profile of [(111)In-DOTA]MG11 in mice. In addition, we compared the efficacy of PA and TO at different administered doses to enhance tumor uptake.
  
  
  METHODS
  [(111)In-DOTA]MG11 was coinjected with (a) vehicle, (b) PA (300 μg), (c) TO (150 μg), (d) Lis (100 μg), (e) PA (300 μg) plus Lis (100 μg), or (f) 30-40 min after intraperitoneal (ip) injection of Race (3 mg) in SCID mice bearing AR42J xenografts. In addition, [(111)In-DOTA]MG11 was coinjected with vehicle, or with progressively increasing amounts of PA (3, 30, or 300 μg) or TO (1.5, 15, and 150 μg) in SCID mice bearing twin A431-CCK2R(+/-) tumors. In all above cases, biodistribution was conducted at 4 h postinjection (pi).
  
  
  RESULTS
  During NEP inhibition, the uptake of [(111)In-DOTA]MG11 in the AR42J tumors impressively increased from 1.8 ± 1.0 % ID/g (controls) to 15.3 ± 4.7 % ID/g (PA) and 12.3 ± 3.6 % ID/g (TO), while with Race tumor values reached 6.8 ± 2.8 % ID/g. Conversely, Lis had no effect on tumor uptake and no additive effect when coinjected with PA. During the dose dependence study in mice, PA turned out to be more efficacious in enhancing tumor uptake of [(111)In-DOTA]MG11 in the CCK2R-positive tumors compared to equimolar amounts of TO. In all cases, renal accumulation remained low, resulting in notable increases of tumor-to-kidney ratios.
  
  
  CONCLUSIONS
  This study has confirmed NEP as the predominant degrading enzyme of [(111)In-DOTA]MG11 and ruled out the involvement of ACE in the in vivo catabolism of the radiotracer. NEP inhibition with the clinically tested NEP inhibitors TO and Race resulted in significant enhancement of tumor-to-kidney ratios vs.
  
  
  METHODS
  However, compared with PA, TO and its prodrug Race induced less potent increases of tumor uptake, highlighting the significance of inhibitor type, administration route, and dose for implementing a first proof-of-principle study in human.
authors.affiliation: Molecular Radiopharmacy, INRASTES, NCSR "Demokritos", Ag. Paraskevi Attikis, GR-153 10, Athens, Greece. katerinakaloudi@yahoo.gr.
authors.forename: Aikaterini
authors.initials: A
authors.lastname: Kaloudi
authors.affiliation: Molecular Radiopharmacy, INRASTES, NCSR "Demokritos", Ag. Paraskevi Attikis, GR-153 10, Athens, Greece. nock_berthold.a@hotmail.com.
authors.forename: Berthold A
authors.initials: BA
authors.lastname: Nock
authors.affiliation: Molecular Radiopharmacy, INRASTES, NCSR "Demokritos", Ag. Paraskevi Attikis, GR-153 10, Athens, Greece. mlymperis@hotmail.com.
authors.forename: Emmanouil
authors.initials: E
authors.lastname: Lymperis
authors.affiliation: Department of Nuclear Medicine, Erasmus MC, 3015 GD, Rotterdam, the Netherlands. r.valkema@erasmusmc.nl.
authors.forename: Roelf
authors.initials: R
authors.lastname: Valkema
authors.affiliation: Department of Nuclear Medicine, Erasmus MC, 3015 GD, Rotterdam, the Netherlands. e.p.krenning@erasmusmc.nl.
authors.forename: Eric P
authors.initials: EP
authors.lastname: Krenning
authors.affiliation: Department of Nuclear Medicine, Erasmus MC, 3015 GD, Rotterdam, the Netherlands. m.hendriks-dejong@erasmusmc.nl.|Department of Radiology, Erasmus MC, 3015 GD, Rotterdam, the Netherlands. m.hendriks-dejong@erasmusmc.nl.
authors.forename: Marion
authors.initials: M
authors.lastname: de Jong
authors.affiliation: Molecular Radiopharmacy, INRASTES, NCSR "Demokritos", Ag. Paraskevi Attikis, GR-153 10, Athens, Greece. maina_thea@hotmail.com.
authors.forename: Theodosia
authors.initials: T
authors.lastname: Maina
keywords: ACE inhibition; Enhancement of tumor uptake; In situ radioligand stabilization; NEP inhibition; Tumor targeting; [111In-DOTA]MG11