Transdermal Detection of MB-102 and Correlation to Meropenem Pharmacokinetics During Continuous Renal Replacement Therapy: In Vivo Results

Critically ill patients undergoing continuous renal replacement therapy (CRRT) have medical conditions requiring extensive pharmacotherapy. Continuous renal replacement therapy impacts drug disposition. Few data exist regarding drug dosing requirements with contemporary CRRT modalities and effluent rates. The practical limitations of pharmacokinetic studies requiring numerous plasma and effluent samples, and lack of generalizability of observations from specific CRRT prescriptions, highlight gaps in bedside assessment of CRRT drug elimination and individualized dosing needs. We employed a porcine model using transdermal fluorescence detection of the glomerular filtration rate fluorescent tracer agent MB-102, with the aim to assess the relationship between systemic exposure of MB-102 and meropenem during CRRT. Animals underwent bilateral nephrectomies and received intravenous bolus doses of MB-102 and meropenem. Once MB-102 equilibrated in the animal, CRRT was initiated. Continuous renal replacement therapy prescriptions comprised four combinations of blood pump (low versus high) and effluent (low versus high) flow rates. Changes in transdermal detected MB-102 clearance occurred immediately with a change in CRRT rates. Blood side meropenem clearance mirrored transdermal MB-102 clearance ( r2 : 0.95-0.97, p value all <0.001). We suggest transdermal MB-102 clearance provides real-time personalized assessment of drug elimination and could optimize prescription of drugs for critically ill patients requiring CRRT.

Adsorption and Clearance of the Novel Fluorescent Tracer Agent MB-102 During Continuous Renal Replacement Therapy: In Vitro Results

MB-102 is a novel fluorescent tracer agent that is exclusively removed from the body by glomerular filtration. This agent can be detected transdermally to provide a real-time measurement of glomerular filtration rate at the point-of-care and is currently in clinical studies for such. MB-102 clearance during continuous renal replacement therapy (CRRT) is unknown. Its plasma protein binding (~0%), molecular weight (~372 Da) and volume of distribution (15-20 L) suggest that it may be removed by renal replacement therapies. To determine the disposition of MB-102 during CRRT, an in vitro study assessing the transmembrane clearance (CL TM ) and adsorptive clearance of MB-102 was conducted. A validated in vitro bovine blood continuous hemofiltration (HF) and continuous hemodialysis (HD) models were performed using two types of hemodiafilters to evaluate CL TM of MB-102. For HF, three different ultrafiltration rates were evaluated. For HD, four different dialysate flow rates were evaluated. Urea was used as a control. No MB-102 adsorption to the CRRT apparatus or either of hemodiafilters was observed. MB-102 is readily removed by HF and HD. Dialysate and ultrafiltrate flow rates directly influence MB-102 CLTM. Hence MB-102 CLTM should be measurable for critically ill patients receiving CRRT.

Evaluation of Developmental and Reproductive Toxicity in Rabbits for MB-102, a Fluorescent Tracer Agent Designed for Real-Time Measurement of Glomerular Filtration Rate

The fluorescent tracer, MB-102, has been designed for the direct, real-time measurement of glomerular filtration rate. Previous studies, both in vitro and in vivo (rats, rabbits and dogs), were conducted to assess potential toxicity including single dose toxicity, mutation assay, chromosomal aberration assay, phototoxicity, local tolerance study, micronuclease assay, hERG channel changes, CNS and cardiovascular safety. The results of these studies led to a safety/toxicology profile for this agent deemed sufficient by the FDA to conduct Phase I and Phase II human clinical studies. In this paper we report on maternal toxicity and the potential effects on embryo-fetal development and the toxicokinetics of MB-102 administered daily via intravenous (bolus) injection into pregnant rabbits during the period of organogenesis gestation day 7-19. Assessment of toxicity was based on mortality, clinical observations, body weight, food consumption, reproductive performance and necropsy and cesarean section findings. Blood samples were collected for toxicokinetic evaluation. No test article findings were noted in any of these studies. The only clinical findings observed were the discoloration of skin, eyes or pelage in the 2 higher dose groups, which were considered related to the color and fluorescent properties of MB-102 and were deemed non-adverse. Exposure, as assessed by C_max and AUC_(0-6) increased in a dose dependent manner from 4.5 to 113 mg/kg/day. No accumulation of the test article was noted after multiple doses were administered. Thus, intravenous administration of MB-102 was not associated with any adverse developmental or reproductive toxicities in pregnant rabbits.

Effectiveness of MB-102, a novel fluorescent tracer agent, for conducting ocular angiography in dogs

OBJECTIVE

To evaluate the effectiveness of a novel fluorescence tracer agent, MB-102, for conducting ocular angiography in dogs.

ANIMALS

10 ophthalmologically normal dogs (2 to 4 years old) and 10 dogs with retinal degeneration or primary open-angle glaucoma (< 6 years old).

PROCEDURES

While anesthetized, all dogs received sodium fluorescein (20 mg/kg, IV) or MB-102 (20 or 40 mg/kg, IV) first and then the other dye in a second treatment session 2 days later in a randomized crossover design. Anterior fluorescence angiography was performed on one eye and posterior fluorescence angiography on the other. Imaging was performed with a full-spectrum camera and camera adaptor system. Filter sets that were tailored to match the excitation and emission characteristics of each angiographic fluorescent agent were used.

RESULTS

All phases and phase intervals during anterior and posterior segment angiography were identified, regardless of the dye used. However, agent fluorescence and visualization of the iridal blood vessels were hindered in some dogs, irrespective of agent, owing to the degree of iridal pigmentation present. No significant difference was noted between the 2 dyes in any phase or phase interval, and slight improvement in image contrast was observed with MB-102 during the venous phases owing to a reduction of vessel wall staining in both normal and diseased eyes.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that MB-102 would be useful for conducting ocular angiography in dogs.

Transdermal fluorescence detection of a dual fluorophore system for noninvasive point-of-care gastrointestinal permeability measurement

The intestinal mucosal barrier prevents macromolecules and pathogens from entering the circulatory stream. Tight junctions in this barrier are compromised in inflammatory bowel diseases, environmental enteropathy, and enteric dysfunction. Dual sugar absorption tests are a standard method for measuring gastrointestinal integrity, however, these are not clinically amenable. Herein, we report on a dual fluorophore system and fluorescence detection instrumentation for which gastrointestinal permeability is determined in a rat small bowel disease model from the longitudinal measured transdermal fluorescence of each fluorophore. This fluorophore technology enables a specimen-free, noninvasive, point-of-care gastrointestinal permeability measurement which should be translatable to human clinical studies.

Transdermal optical renal function monitoring in humans: development, verification, and validation of a prototype device

A prototype medical device for monitoring kidney function by transdermal measurement of the clearance rate of the exogenous fluorescent tracer agent MB-102 (administered intravenously) was developed. Verification of the device with an in vitro protocol is described. The expected renal clearance of the agent was mimicked by preparing a dilution series of MB-102 in the presence of a scattering agent. The slope of a linear fit to the logarithm of fluorescence intensity as a function of dilution step agreed with predictions within 5%, a level of accuracy that would be adequate in assessment of GFR to prevent misdiagnosis of kidney disease. Transdermal measurement was validated using a rat model. A two-compartment pharmacokinetic dependence was observed, with equilibration of the fluorescent agent between the vascular space into which it was injected and the extracellular space into which it subsequently diffused. The best observed signal-to-noise ratios were about 150, allowing determination of the renal clearance time with 5% precision using a 10-min fitting window. Based on the verification and validation methods for transdermal fluorescence detection described herein, the instrument has been approved by the FDA for a first-in-human clinical study, and a first transdermal clearance curve in a human is presented herein.

Measurement of gut permeability using fluorescent tracer agent technology

The healthy gut restricts macromolecular and bacterial movement across tight junctions, while increased intestinal permeability accompanies many intestinal disorders. Dual sugar absorption tests, which measure intestinal permeability in humans, present challenges. Therefore, we asked if enterally administered fluorescent tracers could ascertain mucosal integrity, because transcutaneous measurement of differentially absorbed molecules could enable specimen-free evaluation of permeability. We induced small bowel injury in rats using high- (15 mg/kg), intermediate- (10 mg/kg), and low- (5 mg/kg) dose indomethacin. Then, we compared urinary ratios of enterally administered fluorescent tracers MB-402 and MB-301 to urinary ratios of sugar tracers lactulose and rhamnose. We also tested the ability of transcutaneous sensors to measure the ratios of absorbed fluorophores. Urinary fluorophore and sugar ratios reflect gut injury in an indomethacin dose dependent manner. The fluorophores generated smooth curvilinear ratio trajectories with wide dynamic ranges. The more chaotic sugar ratios had narrower dynamic ranges. Fluorophore ratios measured through the skin distinguished indomethacin-challenged from same day control rats. Enterally administered fluorophores can identify intestinal injury in a rat model. Fluorophore ratios are measureable through the skin, obviating drawbacks of dual sugar absorption tests. Pending validation, this technology should be considered for human use.

Mini review of ultrafast fluorescence polarization spectroscopy [invited]

A mini review is presented on the theory, experiment, and application of the ultrafast fluorescence polarization dynamics and anisotropy with examples of two important medical dyes, namely Indocyanine Green and fluorescein. The time-resolved fluorescence polarization spectra of fluorescent dyes were measured with the excitation of a linearly polarized femtosecond laser pulse, and detected using a streak camera. The fluorescence emitted from the dyes is found to be partially oriented (polarized), and the degree of polarization of emission decreases with time. The decay of the fluorescence component polarized parallel to the excitation beam was found to be faster than that of the perpendicular one. Based on the physical model on the time-resolved polarized emission spectra in nanosecond range first described by Weber [J. Chem. Phys.52, 1654 (1970)], a set of first-order linear differential equations was used to model fluorescence polarization dynamics and anistropy of dye in picoseconds range. Using this model, two important decay parameters were identified separately: the decay rate of total emission intensity and the decay rate of the emission polarization affected by the rotation of fluorescent molecules causing the transfer of emission polarization from one orthogonal component to another. These two decay rates were separated and extracted from the measured time-resolved fluorescence polarization spectra. The emission polarization difference among dyes arising from different molecular volumes was used to enhance the image contrast.

Poly(ethylene oxide)-block-polyphosphester-based Paclitaxel Conjugates as a Platform for Ultra-high Paclitaxel-loaded Multifunctional Nanoparticles

A new type of degradable, nanoscopic polymer assembly containing ultra-high levels of drug loading via covalent attachment within amphiphilic core-shell nanoparticle morphology has been generated as a potentially effective and safe anti-cancer agent. Poly(ethylene oxide)-block-polyphosphoester-based paclitaxel drug conjugates (PEO-b-PPE-g-PTX) were synthesized by rapid, scalable and versatile approach that involves only two steps: organocatalyst-promoted ring-opening-polymerization followed by click reaction-based conjugation of a PTX prodrug. Variations in the polymer-to-PTX stoichiometries allowed for optimization of the conjugation efficiency, the PTX drug loading and the resulting water solubilities of the entire polymer and the PTX content. The PEO-b-PPE-g-PTX formed well-defined micelles in aqueous solution, with a PTX loading capacity as high as 65 wt%, and a maximum PTX concentration of 6.2 mg/mL in water, which is 25000-fold higher than the aqueous solubility of free PTX. The positive cell-killing activity of PEO-b-PPE-g-PTX against several cancer cell lines is demonstrated, and the presence of pendant reactive functionality provides a powerful platform for future work to involve conjugation of multiple drugs and imaging agents to achieve chemotherapy and bioimaging.

Type 1 Phototherapeutic Agents. 2. Cancer Cell Viability and ESR Studies of Tricyclic Diarylamines

Type 1 phototherapeutic agents based on diarylamines were assessed for free radical generation and evaluated in vitro for cell death efficacy in the U937 leukemia cancer cell line. All of the compounds were found to produce copious free radicals upon photoexcitation with UV-A and/or UV-B light, as determined by electron spin resonance (ESR) spectroscopy. Among the diarylamines, the most potent compounds were acridan (4) and 9-phenylacridan (5), with IC50 values of 0.68 μM and 0.17 μM, respectively.

Type 1 phototherapeutic agents, part I: preparation and cancer cell viability studies of novel photolabile sulfenamides

Novel type 1 phototherapeutic agents based on compounds containing S-N bonds (sulfenamides) were synthesized, assessed for free radical generation, and evaluated in vitro for cell death efficacy in four cancer cell lines (U937, HTC11, KB, and HT29). All of the compounds were found to produce copious free radicals upon photoexcitation with UV-A and/or UV-B light, as determined by electron spin resonance spectroscopy. Among the sulfenamides, the most potent compounds were derived from dibenzazepine 7b and dihydroacridine 8b as determined in all of the four cancer cell lines.

Hydrophilic pyrazine dyes as exogenous fluorescent tracer agents for real-time point-of-care measurement of glomerular filtration rate

Various hydrophilic pyrazine-bis(carboxamides) derived from 3,5-diamino-pyrazine-2,5-dicarboxylic acid bearing neutral and anionic groups were prepared and evaluated for use as fluorescent glomerular filtration rate (GFR) tracer agents. Among these, the dianionic d-serine pyrazine derivatives 2d and 2j, and the neutral dihydroxypropyl 2h, exhibited favorable physicochemical and clearance properties. In vitro studies show that 2d, 2h, and 2j have low plasma protein binding, a necessary condition for renal excretion. In vivo animal model results show that these three compounds exhibit a plasma clearance equivalent to iothalamate (a commonly considered gold standard GFR agent). In addition, these compounds have a higher urine recovery compared to iothalamate. Finally, the plasma clearance of 2d, 2h, and 2j remained unchanged upon blockage of the tubular secretion pathway with probenecid, a necessary condition for establishment of clearance via glomerular filtration only. Hence, 2d, 2h, and 2j are promising candidates for translation to the clinic as exogenous fluorescent tracer agents in real-time point-of-care monitoring of GFR.

Multicompartment polymer nanostructures with ratiometric dual-emission pH-sensitivity

Pyrazine-labeled multicompartment nanostructures are shown to exhibit enhanced pH-responsive blue-shifted fluorescence emission intensities compared to their simpler core-shell spherical analogs. An amphiphilic linear triblock terpolymer of ethylene oxide, N-acryloxysuccinimide, and styrene, PEO(45)-b-PNAS(105)-b-PS(45), which lacks significant incompatibility for the hydrophobic block segments and undergoes gradual hydrolysis of the NAS units, underwent supramolecular assembly in mixtures of organic solvent and water to afford multicompartment micelles (MCMs) with a narrow size distribution. The assembly process was followed over time and found to evolve from individual polymer nanodroplets containing internally phase segregated domains, of increasing definition, and ultimately to dissociate into discrete micelles. Upon covalent cross-linking of the MCMs with pH-insensitive pyrazine-based diamino cross-linkers, pH-responsive, photonic multicompartment nanostructures (MCNs) were produced. These MCNs exhibited significant enhancement of overall structural stability, in comparison with the MCMs, and internal structural tunability through the cross-linking chemistry. Meanwhile, the complex compartmentalized morphology exerted unique pH-responsive fluorescence dual-emission properties, indicating promise in ratiometric pH-sensing applications.

Tuning core vs. shell dimensions to adjust the performance of nanoscopic containers for the loading and release of doxorubicin

Detailed studies were performed to probe the effects of the core and shell dimensions of amphiphilic, shell crosslinked, knedel-like polymer nanoparticles (SCKs) on the loading and release of doxorubicin (DOX), a widely-used chemotherapy agent, in aqueous buffer, as a function of the solution pH. Effects of the nanoparticle composition were held constant, by employing SCKs constructed from a single type of amphiphilic diblock copolymer, poly(acrylic acid)-b-polystyrene (PAA-b-PS). A series of four SCK nanoparticle samples, ranging in number-average hydrodynamic diameter from 14-30 nm, was prepared from four block copolymers having different relative block lengths and absolute degrees of polymerization. The ratios of acrylic acid to styrene block lengths ranged from 0.65 to 3.0, giving SCKs with ratios of shell to core volumes ranging from 0.44 to 2.1. Although the shell thicknesses were calculated to be similar (1.5-3.1 nm by transmission electron microscopy (TEM) calculations and 3.5-4.9 nm by small angle neutron scattering (SANS) analyses), two of the SCK nanoparticles had relatively large core diameters (19±2 and 20±2 nm by TEM; 17.4 and 15.3 nm by SANS), while two had similar, smaller core diameters (11±2 and 13±2 nm by TEM; 9.0 and 8.9 nm by SANS). The SCKs were capable of being loaded with 1500-9700 DOX molecules per each particle, with larger numbers of DOX molecules packaged within the larger core SCKs. Their shell-to-core volume ratio showed impact on the rates and extents of release of DOX, with the volume occupied by the poly(acrylic acid) shell relative to the volume occupied by the polystyrene core correlating inversely with the diffusion-based release of DOX. Given that the same amount of polymer was used to construct each SCK sample, SCKs having smaller cores and higher acrylic acid vs. styrene volume ratios were present at higher concentrations than were the larger core SCKs, and gave lower final extents of release., Higher final extents of release and faster rates of release were observed for all DOX-loaded particle samples at pH 5.0 vs. pH 7.4, respectively, ca. 60% vs. 40% at 60 h, suggesting promise for enhanced delivery within tumors and cells. By fitting the data to the Higuchi model, quantitative determination of the kinetics of release was made, giving rate constants ranging from 0.0431 to 0.0540 h⁻¹/² at pH 7.4 and 0.106 to 0.136 h⁻¹/² at pH 5.0. In comparison, the non-crosslinked polymer micelle analogs exhibited rate constants for release of DOX of 0.245 and 0.278 h⁻¹/² at pH 7.4 and 5.0, respectively. These studies point to future directions to craft sophisticated devices for controlled drug release.

Tunable dual-emitting shell-crosslinked nano-objects as single-component ratiometric pH-sensing materials

Dual-emitting photonic nano-objects that can sense changes in the environmental pH are designed based on shell-crosslinked micelles assembled from amphiphilic block copolymers and crosslinked with pH-insensitive chromophores. The chromophoric crosslinkers are tetra-functionalized pyrazine molecules that bear a set of terminal aliphatic amine groups and a set of anilino amine groups, which demonstrate morphology-dependent reactivities towards the poly(acrylic acid) shell domain of the nano-objects. The extent to which the anilino amine groups react with the nano-object shell is shown to affect the hypsochromic shift (blue-shift). The ratio of fluorescence intensity at 496 nm over that of 560 nm is dependent upon the solution pH. We report, herein, observations on the pH-sensitive dual-emission photophysical properties of rod-shaped or spherical nano-objects, whose shell domains offer two distinct platforms for amidation reactions to occur-through formation of activated esters upon addition of carbodiimide or pre-installation of activated ester groups. We demonstrate that physical manipulations (changes in morphology or particle dimensions) or chemical manipulations of the crosslinking reaction (the order of installation of activated esters) lead to fine tuning of dual-emission over ca. 60 nm in a physiologically relevant pH range. Rod-shaped shell-crosslinked nanostructures with poly(p-hydroxystyrene) core show blue-shift as a function of increasing pH while spherical shell-crosslinked nanostructures with polystyrene core and poly(ethylene oxide) corona exhibit blue-shift as a function of decreasing pH.

Photonic Shell-Crosslinked Nanoparticle Probes for Optical Imaging and Monitoring

A pH-insensitive fluorophore is made to give pH-driven responses through its covalent incorporation within a nanostructure derived from pH-responsive polymers. Fluorophore-shell-crosslinked nanoparticles (SCKs) demonstrate notable enhancement of photophysical properties, in the physiological pH region. Fluorophore-SCKs are designed to swell at higher pH and shrink as the pH is lowered, producing high fluorescence vs. low fluorescence outputs, respectively.

Fluorescence-enhanced europium-diethylenetriaminepentaacetic (DTPA)-monoamide complexes for the assessment of renal function

Real-time, noninvasive assessment of glomerular filtration rate (GFR) is essential not only for monitoring critically ill patients at the bedside, but also for staging and monitoring patients with chronic kidney disease. In our pursuit to develop exogenous luminescent probes for dynamic optical monitoring of GFR, we have prepared and evaluated Eu(3+) complexes of several diethylenetriamine pentaacetate (DTPA)-monoamide ligands bearing molecular "antennae" to enhance metal fluorescence via intramolecular ligand-metal fluorescence resonance energy transfer process. The results show that Eu-DTPA-monoamide complex 18b, which contains a quinoxanlinyl antenna, exhibits large (ca. 2700-fold) Eu(3+) fluorescence enhancement. Indeed, complex 18b exhibits the highest fluorescent enhancement observed thus far in the DTPA-type metal complexes. The renal clearance property was assessed using the corresponding radioactive (111)In complex 18a, and the data suggest that this complex clears via a complex mechanism that includes glomerular filtration.

Synergistic effects of light-emitting probes and peptides for targeting and monitoring integrin expression

Integrins mediate many biological processes, including tumor-induced angiogenesis and metastasis. The arginine-glycine-aspartic acid (RGD) peptide sequence is a common recognition motif by integrins in many proteins and small peptides. While evaluating a small library of RGD peptides for imaging alpha(V)beta(3) integrin (ABI)-positive tumor cell line (A549) by optical methods, we discovered that conjugating a presumably inactive linear hexapeptide GRDSPK with a near-infrared carbocyanine molecular probe (Cypate) yielded a previously undescribed bioactive ligand (Cyp-GRD) that targets ABI-positive tumors. MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay with A549 cells showed that Cyp-GRD was not cytotoxic up to 100 muM in cell culture. The compound was internalized by cells, and this internalization was blocked by coincubation with a cyclic RGD peptide (cyclo[RGDfV], f is d-phenylalanine) that binds ABI with high affinity. In vivo, Cyp-GRD selectively accumulated in tumors relative to surrounding normal tissues. Blocking studies with cyclo[RGDfV] inhibited the in vivo uptake of Cyp-GRD, suggesting that both compounds target the same active site of the protein. A strong correlation between the Cyp-GRD peptide and mitochondrial NADH concentration suggests that the new molecule could also report on the metabolic status of cells ex vivo. Interestingly, neither a Cypate-labeled linear RGD peptide nor an (111)In-labeled DOTA-GRD conjugate was selectively retained in the tumor. These results clearly demonstrate the synergistic effects of Cypate and GRD peptide for molecular recognition of integrin expression and suggest the potential of using carbocyanines as optical scaffolds for designing biologically active molecules.

Synthesis, in vitro receptor binding, and in vivo evaluation of fluorescein and carbocyanine peptide-based optical contrast agents

Site-specific delivery of drugs and contrast agents to tumors protects normal tissues from the cytotoxic effects of drugs and enhances the contrast between normal and pathologic tissues. One approach to achieve selectivity is to target overexpressed receptors on the membranes of tumor cells and to visualize the tumors by a noninvasive optical imaging method. Accordingly, we conjugated fluorescein and carbocyanine dyes to somatostatin and bombesin receptor-avid peptides and examined their receptor binding affinities. We also prepared potential dual imaging probes consisting of a bioactive peptide for tumor targeting, a biocompatible dye for optical imaging, and a radioactive or paramagnetic metal chelator for scintigraphic or magnetic resonance imaging of tumors. Using these approaches, the resulting carbocyanine derivatives of somatostatin and bombesin analogues retained high binding for their respective receptors. Further evaluation of representative molecules in rats bearing somatostatin- and bombesin-positive tumors showed selective uptake of the agents by the tumor cells. Unlike carbocyanine derivatives, the receptor binding of fluorescein-somatostatin peptide conjugates was highly sensitive to the type of linker and the site of fluorescein attachment on the nonreceptor binding region of the peptide. In general, the presence of flexible linkers disrupted binding affinity, possibly due to the interaction of the linker's thiourea group with the peptide's cyclic disulfide bond. While the receptor binding affinity of the dual probes was not dependent on the type of chelating group examined, it was affected by the relative positions of fluorescein and chelator on the lysine linker. For somatostatin compounds, best results were obtained when the chelator was on the alpha-amino lysine linker and fluorescein was on the epsilon-amino group. In contrast, conjugation of the chelator to epsilon- and fluorescein to the alpha-amino lysine linker of bombesin peptides resulted in high receptor binding. These findings indicate that despite their small size, conjugation of dyes to truncated somatostatin and bombesin peptide analogues results in promising diagnostic agents that retain high receptor binding activity in vitro. The results further show that these contrast agents can selectively and specifically localize in receptor-positive tumors in rat models.

Novel fluorescent contrast agents for optical imaging of in vivo tumors based on a receptor-targeted dye-peptide conjugate platform

We have designed, synthesized, and evaluated the efficacy of novel dye-peptide conjugates that are receptor specific. Contrary to the traditional approach of conjugating dyes to large proteins and antibodies, we used small peptide-dye conjugates that target over-expressed receptors on tumors. Despite the fact that the peptide and the dye probe have similar molecular mass, our results demonstrate that the affinity of the peptide for its receptor and the dye fluorescence properties are both retained. The use of small peptides has several advantages over large biomolecules, including ease of synthesis of a variety of compounds for potential combinatorial screening of new targets, reproducibility of high purity compounds, diffusiveness to solid tumors, and the ability to incorporate a variety of functional groups that modify the pharmacokinetics of the peptide-dye conjugates. The efficacy of these new fluorescent optical contrast agents was evaluated in vivo in well-characterized rat tumor lines expressing somatostatin (sst(2)) and bombesin receptors. A simple continuous wave optical imaging system was employed. The resulting optical images clearly show that successful specific tumor targeting was achieved. Thus, we have demonstrated that small peptide-dye conjugates are effective as contrast agents for optical imaging of tumors.

Novel receptor-targeted fluorescent contrast agents for in vivo tumor imaging

RATIONALE AND OBJECTIVES

To evaluate the efficacy of a novel tumor receptor-specific small-peptide-near-infrared dye conjugate for tumor detection by optical imaging.

METHODS

A novel, near-infrared dye-peptide conjugate was synthesized and evaluated for tumor-targeting efficacy in a well-characterized rat tumor model (CA20948) known to express receptors for the chosen peptide. A simple continuous-wave optical imaging system, consisting of a near-infrared laser diode, a cooled CCD camera, and an interference filter, was used in this study.

RESULTS

Tumor retention of two non-tumor-specific dyes, indocyanine green and its derivatized analogue, bis-propanoic acid cyanine dye (cypate), was negligible. In contrast, the receptor-specific peptide-cypate conjugate (cytate) was retained in the CA20948 tumor, with an excellent tumor-tonormal-tissue ratio in the six rats examined.

CONCLUSIONS

Optical detection of tumors with a receptor-targeted fluorescent contrast agent has been demonstrated. This result represents a new direction in cancer diagnosis and patient management.

Stabilization of the optical tracer agent indocyanine green using noncovalent interactions

Indocyanine green is a medically useful dye that absorbs and fluoresces in the near infrared and has been sporadically employed clinically as an optical tracer agent for liver function evaluation and cardiac output measurements. The poor stability of this dye in aqueous solution, especially at the high concentrations needed for bolus injection, has been a hindrance in clinical application. However, by using carefully chosen macromolecular additives, the stability of these aqueous dye solutions may be enhanced significantly. Such noncovalent binding between dye and carrier molecules was found to preserve substantially the dye in aqueous solutions for several weeks with no apparent changes in the measured in vivo biological properties.

Polyethyleneglycol-stabilized manganese-substituted hydroxylapatite as a potential contrast agent for magnetic resonance imaging: particle stability in biologic fluids

RATIONALE AND OBJECTIVES

Polymer-stabilized manganese(II)-substituted hydroxylapatite (MnHA) has been investigated as a particulate contrast agent for magnetic resonance imaging. The MnHA core requires a polymer coating to retard opsonization, thereby prolonging its systemic persistence. Therefore, the aim of this study was to assess the stability of various formulations in biologic media in vitro.

METHODS

Polyethyleneglycol-coated manganese(II)-substituted hydroxylapatite particles were studied in bovine plasma as a function of the concentration of polymer in the formulation. Particle sizing techniques and nuclear magnetic resonance proton relaxometry were used to evaluate both in vitro and in vivo stability.

RESULTS

A small-sized particle (approximately 10 nm diameter) that is stable in bovine plasma and rabbit whole blood was formed in formulations with high amounts of polymer concentration. In formulations with low amounts of polymer concentration, larger-sized particles (approximately 100 nm diameter) were present along with the small-sized population. The larger particles de-aggregated into the small-size particle distribution on dispersion in bovine plasma and rabbit whole blood.

CONCLUSIONS

Ultrasmall particles with high surface coat were stable in plasma, whereas larger aggregates de-aggregated. Unlike Mn2+, the interaction of polyethyleneglycol-stabilized manganese(II)-substituted hydroxylapatite with plasma proteins was weak.

Noninvasive fluorescence detection of hepatic and renal function

A noninvasive in vivo fluorescence detection scheme was employed to continuously monitor exogenous dye clearance from the vasculature. Differentiation between normal and impaired physiological function in a rat model was demonstrated for both liver and kidney. A fiber optic transmitted light from source to ear; a second fiber optic positioned near the ear transmitted the fluorescent light to a detector system. Two model dye systems were employed in this initial study. Indocyanine green, known to be exclusively cleared from the blood stream by the liver, was excited in vivo with laser light at 780 nm. The fluorescence signal was detected at 830 nm. A characteristic clearance curve of normal hepatic function was obtained. After a partial hepatectomy of the liver, the clearance curve was extended in time, as would be expected from reduced hepatic function. In addition, fluorescein labeled poly-D-lysine, a small polymer predominantly cleared from the blood stream by the kidney, was excited in vivo with laser light at 488 nm. The fluorescence signal was detected at 518 nm. A characteristic clearance curve of normal renal function was obtained. After a bilateral ligation of the kidneys, the clearance curve remained elevated and constant, indicating little if any clearance. Thus, the feasibility of a new noninvasive method for physiological function assessment was established. © 1998 Society of Photo-Optical Instrumentation Engineers.

Characterization of polyethyleneglycol-stabilized, manganese-substituted hydroxylapatite (MnHA-PEG). A potential MR blood pool agent

PURPOSE

To optimize the performance (or efficacy) of a potential particulate blood pool agent for MR angiography by varying the particle size. The colloidal system under investigation was polyethylene glycol-stabilized manganese-substituted hydroxylapatite (MnHA-PEG).

MATERIAL AND METHODS

Several MnHA-PEG formulations were prepared using various length PEGs (MW = 140-2000). Products were characterized in vitro by dynamic light scattering (DLLS), field flow fractionation (FFF), and relaxometry; and in vivo by blood clearance kinetics in rabbits, and by analytical electron microscopy (EM).

RESULTS

The particle size distribution (PSD) consisted only of small particles (approximately 10-nm diameter) when approximately 40 mo1% PEG was used. At approximately 20 mo1% PEG, larger particles (approximately 100 nm), which are aggregates of the small ones, were also present. The water proton relaxation profiles of the particles in plasma were different from that of the free Mn2+. In plasma, the large aggregates were broken down into the smaller particles which were stable. Although the small particles were efficient relaxation enhancing agents, they were cleared from the blood approximately 3 times faster than the approximately 100-nm diameter aggregates, probably as a consequence of leakage into the extravascular space. Variation of PEG size had no effect on particle characteristics or on blood clearance. Analytical EM of rabbit liver specimens indicated some retention of Mn in mitochondria at the time point when Mn content of other subcellular structures returned to baseline.

CONCLUSION

DLLS and FFF are complementary techniques for sizing particulate MR contrast media. Small MnHA particles are more efficient T1-shortening agents than large ones but they are prone to leakage from the vascular space. Within the MW range explored, the length of PEG molecule had no effect on blood clearance of the MnHA particles. Larger aggregates of MnHA-PEG break down into stable small particles in plasma. Mn clears from the subcellular structures within hepatocytes within 60 min after i.v. MnHA-PEG administration except from the mitochondria in which it appears to accumulate.

Preliminary evaluation of a polyethyleneglycol-stabilized manganese-substituted hydroxylapatite as an intravascular contrast agent for MR angiography

A blood-persistent particulate paramagnetic contrast agent has been formulated via size stabilization of manganese-substituted hydroxylapatite by a polyethylene glycol (PEG) bearing a terminal diphosphonate. At high PEG surface densities (35-40 mol%), particles with mean diameter 8 +/- 2 nm were obtained. Relaxivities of autoclaved samples (at 20 MHz proton Lamor frequency) were R1 = 18.7 +/- .8 mM-1 sec-1 and R2 = 22.3 +/- .7 mM-1 sec-1. The formulation persisted in rabbit blood with a biphasic clearance profile. Half-lives (with amplitudes in parenthesis) were 4 +/- 1 minutes (55%), and 49 +/- 3 minutes (45%), respectively, for the two phases. A dose of 40 mumol Mn/kg body weight enhanced the signal from rabbit vasculature for more than 45 minutes on MR angiograms. Thus, PEG-modified MnHA particles may find use as T1 agents for MR angiography.