Sumeet Dagar

VIP grafted sterically stabilized liposomes for targeted imaging of breast cancer: in vivo studies.

Targeted delivery of radionuclides and therapeutic agents to specific biomarkers of breast cancer has important implications for the diagnosis and therapy of breast cancer. Vasoactive intestinal peptide receptors (VIP-R) are approximately five times more expressed in human breast cancer, compared to normal breast tissue. We have used VIP, a 28 amino acid mammalian neuropeptide, as a breast cancer targeting moiety for targeted imaging of breast cancer. VIP was covalently attached to the surface of sterically stabilized liposomes (SSL) that encapsulated a radionuclide, Tc99m-HMPAO. Rats with n-methyl nitrosourea (MNU)-induced in situ breast cancers were used to test this targeted liposomal imaging agent. Specifically, the pharmacokinetics and biodistribution of Tc99m-HMPAO encapsulating SSL with and without VIP were determined together with their ability to image breast cancer. The presence of VIP did not alter the size and Tc99m-HMPAO encapsulation ability of SSL. It also did not alter the pharmacokinetic profile of SSL. Long-circulating liposomes with and without VIP on their surface accumulated at significantly higher quantities in breast cancer when compared to normal breast, indicating passive targeting of these constructs to cancer tissues. Importantly, in breast cancer, Tc99m-HMPAO encapsulating SSL with VIP showed significantly more accumulation than SSL without VIP. The tumor to non-tumor ratio was also significantly higher for Tc99m-HMPAO encapsulating VIP-SSL than Tc99m-HMPAO encapsulating SSL without VIP, suggesting active targeting of VIP-SSL to breast cancer. Collectively, these data showed that Tc99m-HMPAO encapsulating VIP-SSL can be successfully used for the targeted imaging of breast cancer.

VIP receptors as molecular targets of breast cancer: implications for targeted imaging and drug delivery

Receptors for vasoactive intestinal peptide (VIP-R) are overexpressed in human breast cancer. This phenomenon may have important diagnostic and therapeutic implications because carrier systems loaded with imaging or therapeutic agents, and with surface ligands to VIP-R could potentially be actively targeted to breast cancer. Previously, we have prepared sterically stabilized liposomes (SSL) with VIP non-covalently associated on their surface. However, these liposomes were not able to actively target to breast cancer in rats in situ, most probably due to dissociation of non-covalently associated VIP from SSL. Hence, there is a need to conjugate VIP covalently to SSL. This study aims to begin to address this issue and to test the targeting ability of VIP-SSL to n-methyl nitrosourea (MNU)-induced rat breast cancer in vitro. First, VIP was conjugated to DSPE-PEG(3400)-NHS [1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-n-[poly(ethylene glycol)]-N-hydroxy succinamide, PEG M(w) 3400] under mild conditions to obtain a predominantly 1:1 conjugate of VIP and DSPE-PEG(3400) (DSPE-PEG(3400)-VIP), as evidenced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Next, DSPE-PEG(3400)-VIP was inserted into preformed fluorescent cholesterol (BODIPY-Chol) labeled SSL by incubation at 37 degrees C. To test breast cancer targeting ability in vitro, these VIP-SSL were subsequently incubated with MNU-induced rat breast cancer tissue sections. The results showed that when compared to fluorescent SSL without VIP or non-covalently attached VIP, significantly more VIP-SSL were attached to rat breast cancer tissues indicating that SSL with covalently attached VIP can be actively targeted to rat breast cancer tissues. This targeted carrier system is currently being explored for functional imaging and targeted chemotherapy of breast cancer.

In Vitro Targeting of Acoustically Reflective Immunoliposomes to Fibrin Under Various Flow Conditions

We have previously demonstrated the development of acoustically reflective liposomes as a novel ultrasound contrast agent, that can be conjugated to antibodies for site specific acoustic enhancement of pathologically altered vascular tissue. The liposomes are echogenic due to the lipid composition, without gas entrapment, and have a size of less than one micron (Alkan-Onyuksel et al., 1996). When conjugated to anti-fibrinogen antibodies, the liposomes have the ability to attach to fibrin coated surfaces and thrombi in vitro as demonstrated by scanning electron microscopy and ultrasound imaging (Demos et al., 1997a). Anti-fibrinogen liposomes were shown to attach to fibrous atheroma and thrombi in a Yucatan miniswine model of induced atherosclerosis whereas liposomes conjugated to anti-intercellular adhesion molecule-1 (anti-ICAM-1) were demonstrated to target early stage atherosclerotic plaques (Demos et al., 1997b). The purpose of this study is to evaluate the binding characteristics of anti-fibrinogen liposomes in vitro under a variety of flow conditions in order to optimize the targeting ability of the immunoliposomes. Radiolabeled anti-fibrinogen liposomes were applied to fibrin coated filter paper and placed in a flow circuit under controlled flow conditions. Flow conditions were altered to study the effects of different shear stresses, temperature, plasma flow and pulsatile flow on the retention of liposomes to fibrin after set time periods. The retention of liposomes conjugated to polyclonal and monoclonal antibodies as well as Fab fragments made from monoclonal antibodies were compared. The binding characteristics of liposomes conjugated to different quantities of polyclonal antibodies were analyzed. At physiological shear stress of 1.5 N/m2 (15 dynes/cm2) over 70% of the liposomes remained attached to fibrin after two hours. A smaller and greater portion of the liposomes remained attached at higher and lower shear stresses respectively. Plasma components and temperature had no effect on liposomal retention whereas pulsatile flow resulted in a slight reduction in binding. Monoclonal antibodies showed a slight trend of reduced retention to fibrin over time as compared with polyclonal antibodies and Fab fragments. The quantity of antibody conjugated to the liposomes plays a role in liposome retention as demonstrated by the reduction in liposome retention caused by reducing the quantity of antibody conjugated to the liposomes. Anti-fibrinogen liposomes were retained to the fibrin surface to a large extent under all flow conditions likely to occur in vivo and therefore can provide site specific ultrasound contrast for a long enough time period to allow for imaging after injection.

Detection of VIP receptors in MNU-induced breast cancer in rats : Implications for breast cancer targeting

Vasoactive intestinal peptide (VIP) is a 28 amino acid neuropeptide with a wide range of biological activities. Receptors for VIP (VIP-R) are overexpressed in breast cancer, where they may have diagnostic and therapeutic implications. Although N-methyl nitrosourea (MNU)-induced breast cancer in rats is used extensively as a model to study mammary carcinogenesis, there is no information about the expression of VIP-R in this model. Therefore, the purpose of this study was to investigate the presence of VIP-R in MNU-induced breast cancer in rats so that this model can be used to perform studies involving VIP-R. Breast cancer was induced in 36-day-old virgin female Sprague-Dawley rats, by a single intravenous injection of MNU (50mg/kg body weight). The breast tumors were detected 100–150 days after injection. The normal and cancerous rat breast tissue were excised and 20μ sections were incubated with 40nM fluorescein-labeled VIP (Fluo-VIPTM), in the presence and absence of 1000-fold excess unlabeled VIP, pituitary adenylate cyclase activating polypeptide (PACAP) or secretin. The sections were visualized under a fluorescence microscope and photographed. Fluo-VIPTM stained rat breast cancer tissue homogeneously and to a much greater extent than normal rat breast tissue (p<0.05). This staining was specific as indicated by displacement of Fluo-VIPTM by excess unlabeled VIP and PACAP. Displacement of Fluo-VIPTM by secretin indicated the probable presence of VIP receptors of type VPAC1 (VIP receptor subtype1) in the rat breast. These data suggest that, as in human breast cancer, VIP-R, predominantly of type VPAC1, are overexpressed in MNU-induced rat breast cancer tissue as compared to the normal rat breast tissue. Thus, MNU-induced rat breast cancer model can be used as a tool to study the functional role of VIP-R in human mammary carcinogenesis and VIP-R mediated active breast cancer targeting. This could have implications in the diagnosis, prognosis and therapy of human breast cancer.

Conformation and vasoreactivity of VIP in phospholipids: effects of calmodulin☆

The purpose of this study was to determine the conformation and vasorelaxant effects of vasoactive intestinal peptide (VIP) self-associated with sterically stabilized phospholipid micelles (SSM) and whether calmodulin modulates both of these processes. Circular dichroism spectroscopy revealed that VIP is unordered in aqueous solution at room temperature but assumes appreciable a helix conformation in SSM. This conformational transition was amplified at 37 degrees C and by a low concentration of calmodulin (0.1 nM). Suffusion of VIP in SSM elicited significant time- and concentration-dependent potentiation of vasodilation relative to that elicited by aqueous VIP in the in situ hamster cheek pouch (P < 0.05). This response was significantly potentiated by calmodulin (0.1 nM). Collectively, these data indicate that exogenous calmodulin interacts with VIP in SSM to elicit conformational transition of VIP molecule from a predominantly random coil in aqueous environment to alpha helix in SSM. This process is associated with potentiation and prolongation of VIP-induced vasodilation in the in situ peripheral microcirculation.

Surface-active properties of vasoactive intestinal peptide☆ ☆

The purpose of this study was to determine whether human vasoactive intestinal peptide (VIP) aggregates in aqueous solution and, if so, whether the peptide interacts with a biomimetic phospholipid monolayer and increases surface pressure. Using a custom-made Teflon trough containing HEPES buffer (pH 7.4) at room temperature and a surface tensiometer, we found that the critical micellar concentration (CMC) of VIP is 0.4 microM. Surface pressure of a dipalmitoylphosphatidylcholine (DPPC) monolayer spread over the HEPES buffer declined significantly over 120 min because of phospholipid decomposition. However, injection of VIP at concentrations above CMC into the subphase of the monolayer elicited a significant concentration-dependent increase in surface pressure that persisted for 120 min (P < 0.05). Unlike VIP, injection of [(8)Arg]-vasopressin at an equimolar concentration only prevented the time-dependent decline in DPPC monolayer surface pressure. Taken together, these data indicate that human VIP aggregates in aqueous solution and expresses surface-active properties at physiological concentrations in vitro. We suggest that these attributes could have a role in modulating the bioactive effects of the peptide in vivo.

Human galanin expresses amphipathic properties that modulate its vasoreactivity in vivo

The purpose of this study was to determine whether human galanin, a pleiotropic 30-amino acid neuropeptide, expresses amphipathic properties in vitro and, if so, whether these properties modulate its vasoactive effects in the intact peripheral microcirculation. We found that human galanin aggregates in an aqueous solution and forms micelles with a critical micellar concentration (CMC) of 0.4 microM. In addition, the peptide interacted with model membrane as indicated by long and significant increase of the surface pressure of the biomimetic monolayer membrane in vitro. Interactions of human galanin with sterically stabilized phospholipid micelles (SMM) were not associated with a significant change in peptide conformation. Using intravital microscopy, we found that suffusion of human galanin alone elicited significant concentration-dependent vasoconstriction in the intact hamster cheek pouch. This response was amplified when human galanin in SSM was suffused onto the cheek pouch. The effects of human galanin alone and in SSM were mediated by galanin receptors because galantide, a galanin receptor antagonist, abrogated galanin-induced vasoconstriction. Collectively, these data show that human galanin expresses amphipathic properties in the presence of phospholipids which in turn amplifies its vasoactive effects in the intact peripheral microcirculation.

Interactions of VIP with rigid phospholipid bilayers: implications for vasoreactivity.

The purpose of this study was to determine whether vasoactive intestinal peptide (VIP), a pleiotropic amphipathic peptide, interacts with rigid liposomes composed of gel phase phospholipids. We found that incubation of VIP with small unilamellar gel phase liposomes composed of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and egg phosphatidylglycerol (ePG) for 2h at room temperature had no significant effects on VIP secondary structure. Moreover, suffusion of VIP (0.01, 0.1 and 1.0nmol) incubated in saline or with DPPC/ePG liposomes (size, 30 and 100nm) for 2h at room temperature or 4 degrees C onto the intact hamster cheek pouch microcirculation elicited a similar concentration-dependent vasodilation except for 0.01nmol VIP (P<0.05). By contrast, incubation of VIP with gel phase liposomes overnight at 4 degrees C significantly potentiated vasodilation evoked by all three concentrations of the peptide in comparison to aqueous VIP (P<0.05). VIP-induced vasodilation was liposome size-independent. The ratio of VIP to phospholipids in DPPC/ePG liposomes was concentration-independent. Collectively, these data indicate that short-term interactions of VIP with rigid phospholipid bilayers are limited resulting in only modest effects on VIP vasoreactivity in vivo.

Liposomes in Ultrasound and Gamma Scintigraphic Imaging

Publisher Summary Liposomes are attractive as imaging agents because they can carry high loads of contrast material, and their surface can be modified to be specific to a biomarker of a disease state. The development of sterically stabilized liposomes (SSLs) and a better understanding of the pathophysiology of various disease states have increased the potential use of liposomes for ultrasound and scintigraphic imaging. This suggests that liposomal ultrasound or scintigraphic contrast media can prove to be a better alternative to the existing contrast media for specific applications. Although only one liposomal imaging product is approved by the Food and Drug Administration at this stage, there is reasonable expectation that there would be more clinically and commercially successful liposomal imaging agents in the future. One of the most important developments in liposomal imaging agents is the development of liposomes that are targeted to specific diseases. This can possibly lead to the early and accurate detection of numerous pathologic conditions, such as atherosclerotic plaques and various cancers. Liposomal imaging agents are rapidly being developed for the functional imaging of various disease states.

Liposomal VIP potentiates DNA synthesis in cultured oral keratinocytes

The purpose of this study was to determine whether association of vasoactive intestinal peptide with sterically stabilized liposomes (VIP on SSL) amplifies DNA synthesis evoked by the peptide in cultured chemically transformed hamster oral keratinocytes (HCPC-1) and, if so, whether this response in mediated, in part, by SSL-induced inactivation of neutral endopeptidase 24.11 (NEP; EC 3.4.24.11) and angiotensin I-converting enzyme (ACE; EC 3.4.15.1), two ectoenzymes that modulate HCPC-1 cell growth, in these cells. We found that VIP (10(-9)-10(-6) M) alone elicited a modest, albeit significant, concentration-dependent increase in DNA synthesis in HCPC-1 cells that was maximal after 48-72-h incubation (p < 0.05). VIP on SSL potentiated DNA synthesis in these cells relative to VIP alone. The magnitude of VIP on SSL-induced responses was 1.2-1.6-fold higher than that of VIP alone with maximal effects observed at 10(-9) M and 10(-6) M after 72- and 48-h incubation, respectively. Empty SSL had no significant effects on DNA synthesis. Empty SSL and VIP on SSL had no significant effects on NEP 24.11 and ACE activity in HCPC-1 cells. Collectively, these data indicate that association of VIP with SSL potentiates DNA synthesis in cultured oral keratinocytes relative to VIP alone and that this response is not related to non-specific effects of SSL.