Metabolically stable analogues of substance P: persistent action of partially modified retro-inverso analogues of substance P on rat parotid and hypothalamic slices

Towards Facile Radiolabeling and Preparation of Gallium-68-/Bismuth-213-DOTA-[Thi8, Met(O2)11]-Substance P for Future Clinical Application: First Experiences

Substance P (SP) is a small peptide commonly known as a preferential endogenous ligand for the transmembrane neurokinin-1 receptor. Nuclear Medicine procedures currently involve radiolabeled SP derivatives in peptide radioligand endotherapy of inoperable glioblastoma. Promising clinical results sparked the demand for facile production strategies for a functionalized 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-[Thi⁸, Met(O₂)¹¹]-SP to allow for rapid Gallium-68 or Bismuth-213 complexation. Therefore, we provide a simple kit-like radiotracer preparation method that caters for the gallium-68 activity eluted from a SnO₂ generator matrix as well as preliminary results on the adaptability to produce [²¹³Bi]Bi-DOTA-[Thi⁸, Met(O₂)¹¹]SP from the same vials containing the same starting material. Following a phase of radioanalysis for complexation of gallium-68 to DOTA-[Thi⁸, Met(O₂)¹¹]SP and assessing the radiolabeling parameters, the vials containing appropriate kit-prototype material were produced in freeze-dried batches. The facile radiolabeling performance was tested and parameters for future human application were calculated to meet the criteria for theranostic loco-regional co-administration of activity doses comprising [⁶⁸Ga]Ga-DOTA-[Thi⁸, Met(O₂)¹¹]SP mixed with [²¹³Bi]Bi-DOTA-[Thi⁸, Met(O₂)¹¹]SP. [⁶⁸Ga]Ga-DOTA-[Thi⁸, Met(O₂)¹¹]SP was prepared quantitatively from lyophilized starting material within 25 min providing the required molar activity (18 ± 4 GBq/µmol) and activity concentration (98 ± 24 MBq/mL), radiochemical purity (>95%) and sustained radiolabeling performance (4 months at >95% LE) as well as acceptable product quality (>95% for 120 min). Additionally, vials of the same starting materials were successfully adapted to a labeling strategy available for preparation of [²¹³Bi]Bi-DOTA-[Thi⁸, Met(O₂)¹¹]SP providing sufficient activity for 1-2 human doses. The resultant formulation of [⁶⁸Ga]Ga-/[²¹³Bi]Bi-DOTA-[Thi⁸, Met(O₂)¹¹]SP activity doses was considered of adequate radiochemical quality for administration. This investigation proposes a simple kit-like formulation of DOTA-[Thi⁸, Met(O₂)¹¹]SP-a first-line investigation into a user friendly, straightforward tracer preparation that would warrant efficient clinical investigations in the future. Quantitative radiolabeling was accomplished for [⁶⁸Ga]Ga-DOTA-[Thi⁸, Met(O₂)¹¹]SP and [²¹³Bi]Bi-DOTA-[Thi⁸, Met(O₂)¹¹]SP preparations; a key requirement when addressing the specific route of catheter-assisted co-injection directly into the intratumoral cavities.

Structure-activity relationships of neurokinin A (4-10) at the human tachykinin NK(2) receptor: the role of natural residues and their chirality

A structure-activity study of neurokinin A (NKA) (4-10) was performed to investigate the importance of residue and chirality for affinity and efficacy at the NK(2) receptor in human colon circular muscle. Two series of NKA(4-10) analogues were produced with either L-alanine or the D-enantiomer substituted. Their activities were determined in vitro by means of radioligand binding and isolated smooth muscle pharmacology. NKA was more potent than NKA(4-10) at the human, unlike the rabbit, NK(2) receptor. The contractile response of NKA(4-10) was unaffected by N-terminal acetylation. L-Ala substitution of Asp(4), Val(7), Leu(9), and Met(10) caused an 8- to 80-fold decrease, and substitution of Phe(6) caused a 5000-fold decrease in binding affinity (P < 0.01). Positions Ser(5) and Gly(8) were not significantly affected. In functional studies, a similar pattern was observed. The replacement of residues with their respective D-enantiomer drastically reduced binding affinity and functional potency, particularly at positions 6 and 7 (P < 0.05). NKA(4-10) analogues L-Ala(6), L-Ala(8), D-Phe(6), D-Val(7), and D-Met(10) were partial agonists. An excellent correlation was observed between binding and functional data (r = 0.95). A retro-inverso analogue of NKA(4-10) was inactive. In conclusion, the side chains of Asp(4), Phe(6), Val(7), Leu(9), and Met(10) are structurally important features of NKA(4-10) for agonist activity, and changes in amino acid chirality are detrimental to binding affinity and functional activity. Overall, our data are broadly similar to those of previous studies in the rat. However, at the human NK(2) receptor, unlike the rat, [Ala(8)]NKA(4-10) was an antagonist.

Structure-activity relationship of the ring portion in backbone-cyclic C-terminal hexapeptide analogs of substance P. NMR and molecular dynamics

The conformations of two backbone-cyclized substance P analogs were derived from homo- and heteronuclear NMR measurements and molecular dynamics simulations carried out in DMSO. The analogs contain subtle variations in the ring chemistry and are compared with biologically active analogs previously examined. The correlation between conformation and activity is used to gain insight into the conformational requirements from the pharmacophore.

Synthesis and biological activity of NK-1 selective, N-backbone cyclic analogs of the C-terminal hexapeptide of substance P

The application of the concept of backbone cyclization to linear substance P (SP) analogs is presented. We describe the synthesis, characterization, and biological activity of a series of backbone-to-amino-terminus cyclic analogs of the C-terminal hexapeptide of SP. These analogs were designed on the basis of NMR data and molecular modeling of the selective NK-1 analog WS-septide (Ac[Arg6,Pro9]SP6-11). A series of peptides with the general formula: cyclo[-CH2)m-NH-CO-(CH2)n-CO-Arg-Phe-Phe-N-]-CH2-CO-Leu-Met-NH2 (n = 2, 3, 6 and m = 2, 3, 4) was synthesized by solid phase methodology using Fmoc chemistry for the main chain and Boc chemistry for the building units [Na-(omega-aminoalkyl)Gly] side chains. Cyclization was performed on the resin after removal of the Boc protecting group from the omega-aminoalkyl chain. Cyclic and precyclic analogs were compared. They were purified by HPLC and characterized by mass spectroscopy and NMR. Biological activity and selectivity to the NK-1 neurokinin receptor were found to depend on cyclization and the ring size: The most active and selective analog had a ring of 20 atoms. This analog was found to have enhanced metabolic stability in various tissue preparation compared to WS-septide.

Synthesis of potent agonists of substance P by replacement of Met11 with Glu(OBzl) and N-terminal glutamine with Glp of the C-terminal hexapeptide and heptapeptide of substance P

The analogues [Glp6,Glu(OBzl)11]SP(6-11) and [Glp5,Glu(OBzl)11]SP(5-11) of the C-terminal hexapeptide and heptapeptide of Substance P have been synthesized by conventional solution methods. In each analogue the N-terminal glutamine has been replaced by pyroglutamic acid, while the COOCH2C6H5 ester group has replaced the SCH3 group of the Met11 side chain. The in vitro activity of both analogues has been determined on three biological preparations: guinea pig ileum (GPI), rat vas deferens (RVD) and rat portal vein (RPV). The results showed that both analogues are highly potent and selective agonists on GPI through the NK-1 receptor. They are more potent than SP itself, with 1.54 and 1.25 respective values of relative potency on GPI. Their selectivity has been studied by utilizing atropine-treated guinea pig ileum (GPI+At). The analogues showed low activity on RVD and RPV tissues, which represent NK-2 and NK-3 monoreceptor assay, respectively.

Effect of substance P on rat gastrointestinal transit

The in vivo effect of substance P and related peptide analogs on gastrointestinal transit in unanesthetized rats was studied. Fasted male rats were given intragastrically 0.5 ml of a powdered charcoal (BaSo4.H2O) meal and were concomitantly injected intraperitoneally with 8 micrograms/kg of substance P or a related peptide. In control rats, the percentage of small intestine traversed by the meal 15 min after feeding was 44.9 +/- 1.4 (N = 12). Substance P, [pGlu6]SP, [pGlu6, gPhe8, mGly9]SP and [pGlu5, N-MePhe8, N-MeGly9]SP significantly accelerated intestinal transit: 59.5 +/- 3.1% (N = 7); 66.0 +/- 3.8% (N = 14), 66.8 +/- 2.4% (N = 25), and 58.4 +/- 4.4% (N = 4), respectively. Concomitant injection of [pGlu6]SP and BOC-Phe-Phe-Gly-NHOH, an inhibitor of enzyme degradation at a dose of 800 micrograms/kg lowered by 10-fold the dose of [pGlu6]SP needed to induce the same degree of intestinal transit acceleration. These results indicate that in rats, substance P and related peptides accelerate gastrointestinal transit.

Rapid desensitization of substance P- but not carbachol-induced increases in inositol trisphosphate and intracellular Ca++ in rat parotid acinar cells

The effects of supramaximal concentrations of substance P and the cholinergic agonist carbachol on the accumulation of inositol trisphosphate and the elevation of the intracellular free calcium concentration were compared in rat parotid acinar cells. Substance P was fully as effective as carbachol at initial times, but there was a rapid loss of the substance P responses while the effects of carbachol were well maintained. The loss of the substance P responses represented desensitization rather than degradation of the peptide since further additions of substance P were without effect. Desensitization to substance P did not involve long-term loss of substance P receptors as it was fully reversible in less than twenty minutes, the minimum time to extensively wash previously desensitized cells.