Evading Doxorubicin-Induced Systemic Immunosuppression Using Ultrasound-Responsive Liposomes Combined with Focused Ultrasound.

Doxorubicin (DOX) is a representative anticancer drug with a unique ability to induce immunogenic cell death of cancer cells. However, undesired toxicity on immune cells has remained a significant challenge, hindering the usage of DOX in cancer immunotherapy. Here, we report a combined therapy to avoid the off-target toxicity of DOX by adapting ultrasound-responsive liposomal doxorubicin and focused ultrasound exposure. Histological analysis demonstrated that the combined therapy induced less hemosiderosis of splenocytes and improved tumor infiltration of cytotoxic T lymphocytes. Additionally, in vivo therapeutic evaluation results indicate that the combined therapy achieved higher efficacy when combined with PD-1 immune-checkpoint blockade therapy by improving immunogenicity.

Enhanced delivery to brain using sonosensitive liposome and microbubble with focused ultrasound.

Glioblastoma is considered one of the most aggressive and dangerous brain tumors. However, treatment of GBM has been still challenged due to blood-brain barrier (BBB). BBB prevents that the chemotherapeutic molecules are extravasated to brain. In this study, sonosensitive liposome encapsulating doxorubicin (DOX) was developed for enhancement of GBM penetration in combination with focused ultrasound (FUS) and microbubbles. Upon ultrasound (US) irradiation, microbubbles induce cavitation resulting in the tight junction of BBB endothelium to temporarily open. In addition, the composition of sonosensitive liposome was optimized by comparison of sonosensitivity and intracellular uptake to U87MG cells. The optimal sonosensitive liposome, IMP301-DC, resulted 123.9 ± 38.2 nm in size distribution and 98.2 % in loading efficiency. Related to sonosensitivity of IMP301-DC, US-triggered release ratio of doxorubicin was 69.2 ± 12.3 % at 92 W/cm2 of US intensity for 1 min. In the in vivo experiments, the accumulation of DiD fluorescence probe labeled IMP301-DC-shell in the brain through the BBB opening was increased more than two-fold compared to that of Doxil-shell, non-sonosensitive liposome. US exposure significantly increased GBM cytotoxicity of IMP301-DC. In conclusion, this study demonstrated that IMP301-DC could serve as an alternative solution to enhance the penetration to GBM treatment via BBB opening by non-invasive FUS combined with microbubbles.

Ultrasound-Responsive Liposomes for Targeted Drug Delivery Combined with Focused Ultrasound.

Chemotherapeutic drugs are traditionally used for the treatment of cancer. However, chemodrugs generally induce side effects and decrease anticancer effects due to indiscriminate diffusion and poor drug delivery. To overcome these limitations of chemotherapy, in this study, ultrasound-responsive liposomes were fabricated and used as drug carriers for delivering the anticancer drug doxorubicin, which was able to induce cancer cell death. The ultrasound-sensitive liposome demonstrated a size distribution of 81.94 nm, and the entrapment efficiency of doxorubicin was 97.1 ± 1.44%. The release of doxorubicin under the ultrasound irradiation was 60% on continuous wave and 50% by optimizing the focused ultrasound conditions. In vivo fluorescence live imaging was used to visualize the doxorubicin release in the MDA-MB-231 xenografted mouse, and it was demonstrated that liposomal drugs were released in response to ultrasound irradiation of the tissue. The combination of ultrasound and liposomes suppressed tumor growth over 56% more than liposomes without ultrasound exposure and 98% more than the control group. In conclusion, this study provides a potential alternative for overcoming the previous limitations of chemotherapeutics.

Effects of Lipid Shape and Interactions on the Conformation, Dynamics, and Curvature of Ultrasound-Responsive Liposomes.

We perform coarse-grained molecular dynamics simulations of bilayers composed of various lipids and cholesterol at their different ratios. Simulations show that cholesterol-lipid interactions restrict the lateral dynamics of bilayers but also promote bilayer curvature, indicating that these opposite effects simultaneously occur and thus cannot significantly influence bilayer stability. In contrast, lyso-lipids effectively pack the vacancy in the bilayer composed of cone-shaped lipids and thus reduce bilayer dynamics and curvature, showing that bilayers are more significantly stabilized by lyso-lipids than by cholesterol, in agreement with experiments. In particular, the bilayer composed of cone-shaped lipids shows higher dynamics and curvature than does the bilayer composed of cylindrical-shaped lipids. To mimic ultrasound, a high external pressure was applied in the direction of bilayer normal, showing the formation of small pores that are surrounded by hydrophilic lipid headgroups, which can allow the release of drug molecules encapsulated into the liposome. These findings help to explain experimental observations regarding that liposomes are more significantly stabilized by lyso-lipids than by cholesterol, and that the liposome with cone-shaped lipids more effectively releases drug molecules upon applying ultrasound than does the liposome with cylindrical-shaped lipids.

Combination Therapy with Doxorubicin-Loaded Reduced Albumin Nanoparticles and Focused Ultrasound in Mouse Breast Cancer Xenografts.

Because chemotherapeutic drugs are often associated with serious side effects, the central topic in modern drug delivery is maximizing the localization of drugs at the target while minimizing non-specific drug interactions at unwanted regions. To address this issue, biocompatible nanoparticles have been developed to enhance the drug half-life while minimizing the associated toxicity. Nevertheless, relying solely on the enhanced half-life and enhanced permeability and retention (EPR) effects has been ineffective, and designing stimulus-sensitive nanoparticles to introduce the precise control of drug release has been desired. In this paper, we introduce a pH-sensitive, reduced albumin nanoparticle in combination with focused ultrasound treatment. Not only did these nanoparticles have superior therapeutic efficacy and toxicity profiles when compared to the free drugs in xenograft mouse models, but we were also able to show that the albumin nanoparticles reported in this paper were more suitable than other types of non-reduced albumin nanoparticles as vehicles for drug delivery. As such, we believe that the albumin nanoparticles presented in this paper with desirable characteristics including the induction of strong anti-tumor response, precise control, and superior safety profiles hold strong potential for preclinical and clinical anticancer therapy.

MRI Monitoring of Tumor-Selective Anticancer Drug Delivery with Stable Thermosensitive Liposomes Triggered by High-Intensity Focused Ultrasound.

Monitoring of drug release from a heat-activated liposome carrier provides an opportunity for real-time control of drug delivery and allows prediction of the therapeutic effect. We have developed short-chain elastin-like polypeptide-incorporating thermosensitive liposomes (STLs). Here, we report the development of STL encapsulating gadobenate dimeglumine (Gd-BOPTA), a MRI contrast agent, and doxorubicin (Dox) (Gd-Dox-STL). The Dox release profile from Gd-Dox-STL was comparable to Gd-Dox-LTSL; however, the serum stability of Gd-Dox-STL was much higher than Gd-Dox-LTSL. MRI studies showed that the difference in T1 relaxation time between 37 and 42 °C for Gd-Dox-STL was larger than the difference for Gd-Dox-LTSL. Although relaxivity for both liposomes at 42 °C was similar, the relaxivity of Gd-Dox-STL at 37 °C was 2.5-fold lower than that of Gd-Dox-LTSL. This was likely due to Gd-BOPTA leakage from the LTSL because of low stability at 37 °C. Pharmacokinetic studies showed plasma half-lives of 4.85 and 1.95 h for Gd-Dox-STL and Gd-Dox-LTSL, respectively, consistent with in vitro stability data. In vivo MRI experiments demonstrated corelease of Dox and Gd-BOPTA from STL under mild hyperthermia induced by high-intensity focused ultrasound (HIFU), which suggests STL is a promising tumor selective formulation when coupled with MR-guided HIFU.

Improvement of Antitumor Efficacy by Combination of Thermosensitive Liposome with High-Intensity Focused Ultrasound.

High intensity focused ultrasound (HIFU), allowing for precise heating of the deep and local area, is emerging as the source of mild hyperthermia for delivery of doxorubicin (DOX) using thermosensitive liposomes (TSLs). Conventionally, HIFU has been used for intravascular drug release at tumor tissue by inducing mild hyperthermia immediately upon systemic administration of DOX-TSLs. This immediate heating approach (IHA), however, limits the deep penetration of DOX for high anticancer efficacy. In an attempt to maximize the accumulation of DOX at tumor, the delayed heating approach (DHA) has been explored. In this approach, DOX-TSLs were intravenously administered into the tumor-bearing mice after pre-treatment of tumor tissue with HIFU to increase vascular permeability. We developed the fatty acid-cojugated elastinlike polypeptide bearing TSL (FTSL). The DOX-loaded FTSLs had a hydrodynamic size of 142 nm. In vivo biodistribution study demonstrated that DOX-FTSLs were selectively accumulated at tumor tissue with the maximum amount of DOX at 6 h post-injection. Thereafter, the tumor tissue was heated to 42 °C to induce rapid release of DOX from FTSLs. The results have demonstrated that, compared to IHA, DHA significantly enhances the antitumor efficacy of DOX-FTSLs because of their effective penetration to tumor tissue via the enhanced permeation retention effect, followed by rapid release of DOX.

Formulation optimization and in vivo proof-of-concept study of thermosensitive liposomes balanced by phospholipid, elastin-like polypeptide, and cholesterol.

One application of nanotechnology in medicine that is presently being developed involves a drug delivery system (DDS) employing nanoparticles to deliver drugs to diseased sites in the body avoiding damage of healthy tissue. Recently, the mild hyperthermia-triggered drug delivery combined with anticancer agent-loaded thermosensitive liposomes was widely investigated. In this study, thermosensitive liposomes (TSLs), composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)-2000] (DSPE-PEG), cholesterol, and a fatty acid conjugated elastin-like polypeptide (ELP), were developed and optimized for triggered drug release, controlled by external heat stimuli. We introduced modified ELP, tunable for various biomedical purposes, to our thermosensitive liposome (e-TSL) to convey a high thermoresponsive property. We modulated thermosensitivity and stability by varying the ratios of e-TSL components, such as phospholipid, ELP, and cholesterol. Experimental data obtained in this study corresponded to results from a simulation study that demonstrated, through the calculation of the lateral diffusion coefficient, increased permeation of the lipid bilayer with higher ELP concentrations, and decreased permeation in the presence of cholesterol. Finally, we identified effective drug accumulation in tumor tissues and antitumor efficacy with our optimized e-TSL, while adjusting lag-times for systemic accumulation.

Dynamics and stability of lipid bilayers modulated by thermosensitive polypeptides, cholesterols, and PEGylated lipids.

Lipid bilayers, which consist of dipalmitoylglycerophosphocholines (DPPCs), PEGylated lipids, cholesterols, and elastin-like polypeptides (ELPs; [VPGVG]3) at different molar ratios, were simulated. Simulations were carried out for 2 µs using the coarse-grained (CG) model that had captured the experimentally observed phase behavior of PEGylated lipids and lateral diffusivity of DPPC bilayers. Starting with the initial position of ELPs on the bilayer surface, ELPs insert into the hydrophobic region of the bilayer because of their interaction with lipid tails, consistent with previous all-atom simulations. Lateral diffusion coefficients of DPPCs significantly increase in the bilayer composed of more ELPs and less cholesterols, showing their opposite effects on the bilayer dynamics. In particular, ELPs modulate the dynamics and phase for the disordered liquid bilayer, but not for the ordered gel bilayer, indicating that ELPs can destabilize only the disordered bilayer. In the ordered bilayer, ELP chains tend to have a spherical shape and slowly diffuse, while they are extended and diffuse faster in the disordered bilayer, indicating the effect of the bilayer phase on the conformation and diffusivity of ELPs. These findings explain the experimental observation that the ELP-conjugated liposomes are stable at 310 K (ordered phase) but become unstable and release the encapsulated drugs at 315 K (disordered phase), which suggests the effects of ELPs and cholesterols. Since the cholesterol-stabilized bilayer can be destabilized by the extended shaped ELPs only in the disordered phase (not in the ordered phase), the inclusion of cholesterols is required to safely shield drugs at 310 K as well as allow ELPs to disrupt lipids and destabilize the liposomes at 315 K.

Temperature-triggered tumor-specific delivery of anticancer agents by cRGD-conjugated thermosensitive liposomes.

One of the most effective methods to treat cancer is the specific delivery of anticancer drugs to the target site. To achieve this goal, we designed an anticancer drug with mild hyperthermia-mediated triggering and tumor-specific delivery. To enhance the thermosensitive drug release, we incorporated elastin-like polypeptide (ELP), which is known to be a thermally responsive phase transition peptide into the dipalmitoylphosphatidylcholine (DPPC)-based liposome surface. Additionally, cyclic arginine-glycine-aspartic acid (cRGD) binds to αvß3 integrin, which is overexpressed in angiogenic vasculature and tumor cells, was introduced on the liposome. ELP-modified liposomes with the cRGD targeting moiety were prepared using a lipid film hydration method, and doxorubicin (DOX) was loaded into the liposome by the ammonium sulfate-gradient method. The cRGD-targeted and ELP-modified DOX-encapsulated liposomes (RELs) formed spherical vesicles with a mean diameter of 181 nm. The RELs showed 75% and 83% DOX release at 42°C and 45°C, respectively. The stability of RELs was maintained up to 12h without the loss of their thermosensitive function for drug release. Flow cytometry results showed that the cellular uptake of DOX in RELs into αvß3 integrin-overexpressing U87MG and HUVEC cells was 8-fold and 10-fold higher, respectively, than that of non-targeting liposomes. Confocal microscopy revealed that REL released DOX only under the mild hyperthermia condition at 42°C by showing the localization of DOX in nuclei and the liposomes in the cytosol. The cell cytotoxicity results demonstrated that REL can efficiently kill U87MG cells through cRGD targeting and thermal triggering. The in vivo tumoral accumulation measurement showed that the tumor-targeting effect of RELs was 5-fold higher than that of non-targeting liposomes. This stable, target-specific, and thermosensitive liposome shows promise to enhance therapeutic efficacy if it is applied along with a relevant external heat-generating medical system.

Bioreducible hyaluronic acid conjugates as siRNA carrier for tumor targeting.

The successful clinical translation of siRNA-based therapeutics requires efficient carrier systems that can specifically deliver siRNA within the cytosol of the target cells. Although numerous polymeric nanocarriers forming ionic complexes with siRNA have been investigated for cancer therapy, their poor stability and lack of tumor targetability have impeded their in vivo applications. To surmount these limitations, we synthesized a novel type of biodegradable hyaluronic acid-graft-poly(dimethylaminoethyl methacrylate) (HPD) conjugate that can form complexes with siRNA and be chemically crosslinked via the formation of the disulfide bonds under facile conditions. The crosslinked siRNA-HPD (C-siRNA-HPD) complexes exhibited high stability in a 50% serum solution, as compared to the uncrosslinked siRNA-HPD (U-siRNA-HPD) complexes and free siRNA. Both the C-siRNA-HPD and U-siRNA-HPD complexes were efficiently taken up by the CD44-overexpressing melanoma cells (B16F10), but not by the normal fibroblast cells (NIH3T3). When the RFP-expressing B16F10 cells were treated with the complexes or free siRNA, the C-siRNA-HPD complexes showed the highest decrease in RFP expression. In vivo studies demonstrated the selective accumulation of C-siRNA-HPD complexes at the tumor site after their systemic administration into tumor-bearing mice, resulting in an efficient gene silencing effect. Overall, these results suggest that the HPD conjugate could be used as an efficient carrier for the tumor-targeted delivery of siRNA.

Novel temperature-triggered liposome with high stability: formulation, in vitro evaluation, and in vivo study combined with high-intensity focused ultrasound (HIFU).

We developed a novel temperature-sensitive liposome, STL composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)-2000] (DSPE-PEG-2000), cholesterol, and a fatty acid conjugated elastin-like polypeptide (ELP). The STL had a unilamellar spherical shape with a mean diameter of 160 nm. Doxorubicin (DOX) was encapsulated by the STL using an ammonium sulfate gradient method with a lipid to drug ratio of 1:0.2 (w/w), resulting in 95% loading efficiency. The STL exhibited better stability than conventional low temperature sensitive liposome (LTSL-lysolipid-based temperature sensitive liposomes; DPPC:MSPC:DSPE-PEG-2000=90:10:4) at 37 °C in the presence of serum; there was rapid release of doxorubicin in the range of 39-42 °C (≥95% release at 42 °C within 10s). A confocal microscope revealed that DOX encapsulated in STL (STL-DOX) was taken up much better by cell nuclei at 42 °C than at 37 °C. The difference in cell viability between 37 and 42 °C was 63% relative to STL-DOX and 18% for LTSL-DOX. The pharmacokinetics (PK) and antitumor effect of STL-DOX combined with high-intensity focused ultrasound (HIFU) were studied, and compared with LTSL. An in vivo study demonstrated that STL-DOX is highly stable, with a long circulating property (half life=2.03±0.77 h) in HIFU-untreated mice, and resulted in significant tumor regression for 2 days after intravenous injection of STL-DOX at 5 mg DOX/kg in combination with HIFU. These results are better than conventional LTSL, for which the blood circulation time is short (0.92±0.17 h) and inhibition of tumor growth is weak. These results indicate that the properties of stability at 37 °C and burst release at 42 °C of STL-DOX act synergistically against tumors.

Effect of arginine-rich peptide length on the structure and binding strength of siRNA-peptide complexes.

Heparin decomplexation experiments, as well as all-atom (AA) and coarse-grained (CG) molecular dynamics (MD) simulations, were performed to determine the effect of the size of arginine(Arg)-rich peptides on the structure and binding strength of the siRNA-peptide complex. At a fixed peptide/siRNA mole ratio of 5:1 or 10:1, the siRNA complexes with peptides longer than nine Arg residues are more easily decomplexed by heparin than are those with nine Arg residues. At these mole ratios, peptides longer than nine Arg residues have cationic/anionic charge ratios in excess of unity, and produce more weakly bound complexes than nine Arg residue ones do. AA simulations of mixtures of peptides with a single siRNA show formation of an electrostatically induced complex, and the longer peptides produce a larger complex, but with no significant increase in the number of Arg residues bound to the siRNA. Larger-scale CG-MD simulations show that multiple siRNAs can be linked together by peptides into a large complex, as observed in the experiments. The peptides longer than nine residues, which at mole ratio 5:1 yield a peptide/siRNA charge ratio in excess of unity, include many noninteracting Arg residues, which repel each other electrostatically. This leads to a less dense complex than for 9-residue peptides, which can explain why these longer complexes are more easily decomplexed by heparin molecules, as observed in the experiments. The key role of the charge ratio is supported by simulations that show that, at a mole ratio of 2.5 peptides per siRNA, the longer 18-residue peptide has a charge ratio of roughly unity and also shows a tight complex, just as the 9-residue peptide does at a 5:1 mole ratio, where its charge ratio is also unity.

Robust PEGylated hyaluronic acid nanoparticles as the carrier of doxorubicin: mineralization and its effect on tumor targetability in vivo.

The in vivo stability and tumor targetability of self-assembled polymeric nanoparticles are crucial for effective drug delivery. In this study, to develop biostable nanoparticles with high tumor targetability, poly(ethylene glycol)-conjugated hyaluronic acid nanoparticles (PEG-HANPs) were mineralized through controlled deposition of inorganic calcium and phosphate ions on the nanoparticular shell via a sequential addition method. The resulting nanoparticles (M-PEG-HANPs) had a smaller size (153.7±4.5nm) than bare PEG-HANPs (265.1±9.5nm), implying that mineralization allows the formation of compact nanoparticles. Interestingly, when the mineralized nanoparticles were exposed to acidic buffer conditions (

Cationic solid lipid nanoparticles reconstituted from low density lipoprotein components for delivery of siRNA.

Cationic solid lipid nanoparticles (SLN), reconstituted from natural components of protein-free low-density lipoprotein, were used to deliver small interfering RNA (siRNA). The cationic SLN was prepared using a modified solvent-emulsification method. The composition was 45% (w/w) cholesteryl ester, 3% (w/w) triglyceride, 10% (w/w) cholesterol, 14% (w/w) dioleoylphosphatidylethanolamine (DOPE), and 28% (w/w) 3beta-[ N-(N',N'-dimethylaminoethane)carbamoyl]-cholesterol (DC-chol). The SLN had a mean diameter of 117+/-12 nm and a surface zeta potential value of +41.76+/-2.63 mV. A reducible conjugate of siRNA and polyethylene glycol (PEG) (siRNA-PEG) was anchored onto the surface of SLN via electrostatic interactions, resulting in stable complexes in buffer solution and in even 10% serum. Under an optimal weight ratio of DC-chol of SLN and siRNA-PEG conjugate, the complexes exhibited higher gene silencing efficiency of GFP and VEGF than that of polyethylenimine (PEI) 25K with showing much reduced cell cytotoxicity. Flow cytometry results also showed that siRNA-PEG/SLN complexes were efficiently taken up by cells. Surface-modified and reconstituted protein-free LDL mimicking SLN could be utilized as noncytotoxic, serum-stable, and highly effective carriers for delivery of siRNA.

Analysis of plasma protein adsorption onto PEGylated nanoparticles by complementary methods: 2-DE, CE and Protein Lab-on-chip system.

The biodistribution of colloidal carriers after their administration in vivo depends on the adsorption of some plasma proteins and apolipoproteins on their surface. Poly(methoxypolyethyleneglycol cyanoacrylate-co-hexadecylcyanoacrylate) (PEG-PHDCA) nanoparticles have demonstrated their capacity to cross the blood-brain barrier (BBB) by a mechanism of endocytosis. In order to clarify this mechanism at the molecular level, proteins and especially apolipoproteins adsorbed at the surface of PEG-PHDCA nanoparticles were analyzed by complementary methods such as CE and Protein Lab-on-chip in comparison with 2-D PAGE as a method of reference. Thus, the ability of those methodologies to identify and quantify human and rat plasma protein adsorption onto PEG-PHDCA nanoparticles and conventional PHDCA nanoparticles was evaluated. The lower adsorption of proteins onto PEG-PHDCA nanoparticles comparatively to PHDCA nanoparticles was evidenced by 2-D PAGE and Protein Lab-on-chip methods. CE allowed the quantification of adsorbed proteins without the requirement of a desorption procedure but failed, in this context, to analyze complex mixtures of proteins. The Protein Lab-on-chip method appeared to be very useful to follow the kinetic of protein adsorption from serum onto nanoparticles; it was complementary to 2-D PAGE which allowed the identification (with a relative quantification) of the adsorbed proteins. The overall results suggest the implication of the apolipoprotein E in the mechanism of passage of PEG-PHDCA nanoparticles through the BBB.

A methodology to study intracellular distribution of nanoparticles in brain endothelial cells.

Cell internalisation and intracellular distribution of PEG-coated polyhexadecylcyanoacrylate (PEG-PHDCA) nanoparticles in rat brain endothelial cells (RBEC) have been investigated. A cell fractionation method has been developed based on the selective permeabilisation of RBEC plasma membrane by digitonin. By interacting with membrane cholesterol, digitonin creates pores allowing the release of soluble and diffusible species outside the cell. The selectivity of plasma membrane permeabilisation was controlled by using compartment markers such as lactate dehydrogenase (LDH) for cytoplasm and cathepsin B for lysosomes. An optimal digitonin concentration of 0.003% (w/v) has been identified to induce a pattern of membrane permeabilisation corresponding to the extraction of 72% LDH and less than 15% of Cathepsin B. Membrane permeabilisation at this digitonin concentration allows one to distinguish between the cell cytoplasm and its endo/lysosomal fraction. This methodology was applied to investigate the intracellular distribution of the nanoparticles after their incubation with the RBEC. The results showed that PEG-PHDCA nanoparticles were able to be internalised to a higher extent than PHDCA nanoparticles (after 20 min incubation). Additionally, these nanoparticles displayed different patterns of intracellular capture, depending on their specific surface composition: PEG-PHDCA nanoparticles were 48% in the plasma membrane, 24% in the cytoplasm, 20% in vesicular compartments and 8% associated with the fraction of the nucleus, the cytoskeleton and caveolae suggesting that PEG-PHDCA nanoparticle uptake by RBEC is specific and presumably due to endocytosis. Confocal microscopy studies confirmed the cellular uptake of PEG-PHDCA nanoparticles.

The two NK-1 binding sites are distinguished by one radiolabelled substance P analogue.

Two non-stoichiometric binding sites had previously been characterized for the NK-1 receptor using two different types of radiolabelled analogues of substance P. However, the question remained on their eventual conformational interconversion induced or not by the ligand. In this study, kinetic, saturation, and competition studies using [3H]propionyl[Pro(9)]SP demonstrate the existence of two independent binding components in CHO cells transfected with the human NK-1 receptor, with K(d) values of 0.040 nM ( approximately 20% of total sites) and 5.9 nM ( approximately 80% of total sites) that correspond to those of the two previously described binding sites. These two binding sites do not seem to interconvert since the minor one can be selectively extinguished in saturation studies in the presence of a SP analogue specific of this binding site.