Molecular imaging of apoptosis: from micro to macro

Apoptosis, or programmed cell death, is involved in numerous human conditions including neurodegenerative diseases, ischemic damage, autoimmune disorders and many types of cancer, and is often confused with other types of cell death. Therefore strategies that enable visualized detection of apoptosis would be of enormous benefit in the clinic for diagnosis, patient management, and development of new therapies. In recent years, improved understanding of the apoptotic machinery and progress in imaging modalities have provided opportunities for researchers to formulate microscopic and macroscopic imaging strategies based on well-defined molecular markers and/or physiological features. Correspondingly, a large collection of apoptosis imaging probes and approaches have been documented in preclinical and clinical studies. In this review, we mainly discuss microscopic imaging assays and macroscopic imaging probes, ranging in complexity from simple attachments of reporter moieties to proteins that interact with apoptotic biomarkers, to rationally designed probes that target biochemical changes. Their clinical translation will also be our focus.

(18)F-alfatide II and (18)F-FDG dual-tracer dynamic PET for parametric, early prediction of tumor response to therapy

A single dynamic PET acquisition using multiple tracers administered closely in time could provide valuable complementary information about a tumor's status under quasiconstant conditions. This study aimed to investigate the utility of dual-tracer dynamic PET imaging with (18)F-alfatide II ((18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2) and (18)F-FDG for parametric monitoring of tumor responses to therapy.

METHODS

We administered doxorubicin to one group of athymic nude mice with U87MG tumors and paclitaxel protein-bound particles to another group of mice with MDA-MB-435 tumors. To monitor therapeutic responses, we performed dual-tracer dynamic imaging, in sessions that lasted 90 min, starting with injection via the tail vein catheters with (18)F-alfatide II, followed 40 min later by (18)F-FDG. To achieve signal separation of the 2 tracers, we fit a 3-compartment reversible model to the time-activity curve of (18)F-alfatide II for the 40 min before (18)F-FDG injection and then extrapolated to 90 min. The (18)F-FDG tumor time-activity curve was isolated from the 90-min dual-tracer tumor time-activity curve by subtracting the fitted (18)F-alfatide II tumor time-activity curve. With separated tumor time-activity curves, the (18)F-alfatide II binding potential (Bp = k3/k4) and volume of distribution (VD) and (18)F-FDG influx rate ((K1 × k3)/(k2 + k3)) based on the Patlak method were calculated to validate the signal recovery in a comparison with 60-min single-tracer imaging and to monitor therapeutic response.

RESULTS

The transport and binding rate parameters K1-k3 of (18)F-alfatide II, calculated from the first 40 min of the dual-tracer dynamic scan, as well as Bp and VD correlated well with the parameters from the 60-min single-tracer scan (R(2) > 0.95). Compared with the results of single-tracer PET imaging, (18)F-FDG tumor uptake and influx were recovered well from dual-tracer imaging. On doxorubicin treatment, whereas no significant changes in static tracer uptake values of (18)F-alfatide II or (18)F-FDG were observed, both (18)F-alfatide II Bp and (18)F-FDG influx from kinetic analysis in tumors showed significant decreases. For therapy of MDA-MB-435 tumors with paclitaxel protein-bound particles, a significant decrease was observed only with (18)F-alfatide II Bp value from kinetic analysis but not (18)F-FDG influx.

CONCLUSION

The parameters fitted with compartmental modeling from the dual-tracer dynamic imaging are consistent with those from single-tracer imaging, substantiating the feasibility of this methodology. Even though no significant differences in tumor size were found until 5 d after doxorubicin treatment started, at day 3 there were already substantial differences in (18)F-alfatide II Bp and (18)F-FDG influx rate. Dual-tracer imaging can measure (18)F-alfatide II Bp value and (18)F-FDG influx simultaneously to evaluate tumor angiogenesis and metabolism. Such changes are known to precede anatomic changes, and thus parametric imaging may offer the promise of early prediction of therapy response.

Assessment and comparison of magnetic nanoparticles as MRI contrast agents in a rodent model of human hepatocellular carcinoma

The purpose of this study was to synthesize, characterize and tailor the surface properties of magnetic nanoparticles with biocompatible copolymer coatings and to evaluate the efficiency of the resulting nanoconjugates as magnetic resonance imaging (MRI) contrast agents for liver imaging. Magnetic nanoparticles with core diameters of 10 and 30 nm were synthesized by pyrolysis and were subsequently coated with a copolymer containing either carboxyl (SHP) or methoxy groups as termini. All four formulas, and ferumoxides (Feridex I.V.(®)), were individually injected intravenously into separate, normal Balb/C mice (at 2.5, 1.0 and 0.56 mg Fe kg(-1)), and the animals underwent T(2)-weighted MRI at multiple time points post injection (p.i.) to evaluate the hepatic uptake and clearance. Furthermore, we compared the abilities of the new formulas and Feridex to detect tumors in an orthotropic Huh7 tumor model. Transmission electron microscopy (TEM) revealed a narrow size distribution of both the 10 and 30 nm nanoparticles, in contrast to a wide size distribution of Feridex. MTT, apoptosis and cyclin/DNA flow cytometry assays showed that the polymer coated nanoparticles had no adverse effect on cell growth. Among all the tested formulas, including Feridex, SHP-30 showed the highest macrophage uptake at the in vitro level. In vivo MRI studies on normal mice confirmed the superiority of SHP-30 in inducing hypointensities in the liver tissue, especially at clinical dose (0.56 mg Fe kg(-1)) and 3 T field. SHP-30 showed better contrast-to-noise ratio than Feridex on the orthotropic Huh7 tumor model. SHP-30 was found to be an efficient contrast agent for liver MR imaging. The success of this study suggests that, by improving the synthetic approach and by tuning the surface properties of IONPs, one can arrive at better formulas than Feridex for clinical practice.

Electrochemical immunosensors for detection of cancer protein biomarkers

Bioanalytical methods have experienced unprecedented growth in recent years, driven in large part by the need for faster, more sensitive, more portable ("point of care") systems to detect protein biomarkers for clinical diagnosis. Electrochemical detection strategies, used in conjunction with immunosensors, offer advantages because they are fast, simple, and low cost. Recent developments in electrochemical immunosensors have significantly improved the sensitivity needed to detect low concentrations of biomarkers present in early stages of cancer. Moreover, the coupling of electrochemical devices with nanomaterials, such as gold nanoparticles, carbon nanotubes, magnetic particles, and quantum dots, offers multiplexing capability for simultaneous measurements of multiple cancer biomarkers. This review will discuss recent advances in the development of electrochemical immunosensors for the next generation of cancer diagnostics, with an emphasis on opportunities for further improvement in cancer diagnostics and treatment monitoring. Details will be given for strategies to increase sensitivity through multilabel amplification, coupled with high densities of capture molecules on sensor surfaces. Such sensors are capable of detecting a wide range of protein quantities, from nanogram to femtogram (depending on the protein biomarkers of interest), in a single sample.

Peptides in cancer nanomedicine: drug carriers, targeting ligands and protease substrates

Peptides are attracting increasing attention as therapeutic agents, as the technologies for peptide development and manufacture continue to mature. Concurrently, with booming research in nanotechnology for biomedical applications, peptides have been studied as an important class of components in nanomedicine, and they have been used either alone or in combination with nanomaterials of every reported composition. Peptides possess many advantages, such as smallness, ease of synthesis and modification, and good biocompatibility. Their functions in cancer nanomedicine, discussed in this review, include serving as drug carriers, as targeting ligands, and as protease-responsive substrates for drug delivery.

Functional MnO nanoclusters for efficient siRNA delivery

A non-viral gene delivery nanovehicle based on Alkyl-PEI2k capped MnO nanoclusters was synthesized via a simple, facile method and used for efficient siRNA delivery and magnetic resonance imaging.

Surface-engineered magnetic nanoparticle platforms for cancer imaging and therapy

Enormous efforts have been made toward the translation of nanotechnology into medical practice, including cancer management. Generally the applications have fallen into two categories: diagnosis and therapy. Because the targets are often the same, the development of separate approaches can miss opportunities to improve efficiency and effectiveness. The unique physical properties of nanomaterials enable them to serve as the basis for superior imaging probes to locate and report cancerous lesions and as vehicles to deliver therapeutics preferentially to those lesions. These technologies for probes and vehicles have converged in the current efforts to develop nanotheranostics, nanoplatforms with both imaging and therapeutic functionalities. These new multimodal platforms are highly versatile and valuable components of the emerging trend toward personalized medicine, which emphasizes tailoring treatments to the biology of individual patients to optimize outcomes. The close coupling of imaging and treatment within a theranostic agent and the data about the evolving course of an illness that these agents provide can facilitate informed decisions about modifications to treatment. Magnetic nanoparticles, especially superparamagnetic iron oxide nanoparticles (IONPs), have long been studied as contrast agents for magnetic resonance imaging (MRI). Owing to recent progress in synthesis and surface modification, many new avenues have opened for this class of biomaterials. Such nanoparticles are not merely tiny magnetic crystals, but potential platforms with large surface-to-volume ratios. By taking advantage of the well-developed surface chemistry of these materials, researchers can load a wide range of functionalities, such as targeting, imaging and therapeutic features, onto their surfaces. This versatility makes magnetic nanoparticles excellent scaffolds for the construction of theranostic agents, and many efforts have been launched toward this goal. In this Account, we introduce the surface engineering techniques that we and others have developed, with an emphasis on how these techniques affect the role of nanoparticles as imaging or therapeutic agents. We and others have developed a set of chemical methods to prepare magnetic nanoparticles that possess accurate sizes, shapes, compositions, magnetizations, relaxivities, and surface charges. These features, in turn, can be harnessed to adjust the toxicity and stability of the nanoparticles and, further, to load functionalities, via various mechanisms, onto the nanoparticle surfaces.

Improvement of CXCR4 tracer specificity for PET imaging

Tumors expressing the chemokine receptor CXCR4 have been reported to be more aggressive and to produce more metastatic seeding in specific organs, such as the bone marrow. However, evaluation of tumors for CXCR4 expression requires testing of ex vivo biopsy samples, and is not routinely done in cancer management. In prior work to address this issue, we and others have developed tracers for positron emission tomography (PET) that targeted CXCR4, but in addition to binding to CXCR4 these tracers also bound to red blood cells (and to other unrelated targets) in vivo. Here we report two new tracers based on the CXCR4 peptide antagonist 4F-benzoyl-TN14003 (T140) that bind to CXCR4, but not to undesired targets. These tracers, NOTA-NFB and DOTA-NFB, show slight reductions in both 1) binding affinities for CXCR4 and 2) inhibition of CXCL12 induced migration, compared to T140, in vitro. Both NOTA-NFB and DOTA-NFB specifically accumulate in CXCR4-positive, but not CXCR4-negative, tumor xenografts in mice and allow clear visualization of CXCR4 expression by PET. Evaluation of NOTA-NFB and DOTA-NFB for their potential to mobilize immune cells and progenitor cells from the bone marrow to the peripheral blood revealed slightly reduced, but still comparable, results to the parent molecule T140. The tracers reported here may allow the evaluation of CXCR4 expression in primary tumors and metastatic nodules, and enable better informed, more personalized treatment for patients with cancer.

HSA coated iron oxide nanoparticles as drug delivery vehicles for cancer therapy

An ongoing effort in the field of nanomedicine is to develop nanoplatforms with both imaging and therapeutic functions, the "nanotheranostics". We have previously developed a human serum albumin (HSA) coated iron oxide nanoparticle (HINP) formula and used multiple imaging modalities to validate its tumor targeting attributes. In the current study, we sought to impart doxorubicin (Dox) onto the HINPs and to assess the potential of the conjugates as theranostic agents. In a typical preparation, we found that about 0.5 mg of Dox and 1 mg of iron oxide nanoparticles (IONPs, Fe content) could be loaded into 10 mg of HSA matrices. The resulting D-HINPs (Dox loaded HINPs) have a hydrodynamic size of 50 nm and are able to release Dox in a sustained fashion. More impressively, the HINPs can assist the translocation of Dox across the cell membrane and even its accumulation in the nucleus. In vivo, D-HINPs retained a tumor targeting capability of HINPs, as manifested by both in vivo MRI and ex vivo immunostaining results. In a follow-up therapeutic study on a 4T1 murine breast cancer xenograft model, D-HINPs showed a striking tumor suppression effect that was comparable to that of Doxil and greatly outperformed free Dox. Such a strategy can be readily extended to load other types of small molecules, making HINP a promising theranostic nanoplatform.

Multimodality imaging of tumor response to doxil

Purpose: Early assessment of tumor responses to chemotherapy could enhance treatment outcomes by ensuring that, from the beginning, treatments meet the individualized needs of patients. In this study, we applied multiple modality molecular imaging techniques to pre-clinical monitoring of early tumor responses to Doxil, focusing on imaging of apoptosis.

Methods: Mice bearing UM-SCC-22B human head and neck squamous cancer tumors received either PBS or 1 to 2 doses of Doxil® (doxorubicin HCl liposome injection) (10 mg/kg/dose). Bioluminescence signals from an apoptosis-responsive reporter gene were captured for apoptosis evaluation. Tumor metabolism and proliferation were assessed by ¹⁸F-FDG and 3'-¹⁸F-fluoro-3'-deoxythymidine (¹⁸F-FLT) positron emission tomography. Diffusion-weighted magnetic resonance imaging (DW-MRI) was performed to calculate averaged apparent diffusion coefficients (ADCs) for the whole tumor volume. After imaging, tumor samples were collected for histological evaluation, including terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), anti-CD31, and Ki-67 immunostaining.

Results: Two doses of Doxil significantly inhibited tumor growth. Bioluminescence imaging (BLI) indicated apoptosis of tumor cells after just 1 dose of Doxil treatment, before apparent tumor shrinkage. ¹⁸F-FDG and ¹⁸F-FLT PET imaging identified decreased tumor metabolism and proliferation at later time points than those at which BLI indicated apoptosis. MRI measurements of ADC altered in response to Doxil, but only after tumors were treated with 2 doses. Decreased tumor proliferation and increased apoptotic cells were confirmed by changes of Ki-67 index and apoptotic ratio.

Conclusion: Our study of tumor responses to different doses of Doxil demonstrated that it is essential to combine apoptosis imaging strategies with imaging of other critical biological or pathological pathways, such as metabolism and proliferation, to improve clinical decision making in apoptosis-related diseases and interventions.

Core body temperature in obesity

BACKGROUND

A lower core body temperature set point has been suggested to be a factor that could potentially predispose humans to develop obesity.

OBJECTIVE

We tested the hypothesis that obese individuals have lower core temperatures than those in normal-weight individuals.

DESIGN

In study 1, nonobese [body mass index (BMI; in kg/m(2)) <30] and obese (BMI ≥30) adults swallowed wireless core temperature-sensing capsules, and we measured core temperatures continuously for 24 h. In study 2, normal-weight (BMI of 18-25) and obese subjects swallowed temperature-sensing capsules to measure core temperatures continuously for ≥48 h and kept activity logs. We constructed daily, 24-h core temperature profiles for analysis.

RESULTS

Mean (±SE) daily core body temperature did not differ significantly between the 35 nonobese and 46 obese subjects (36.92 ± 0.03°C compared with 36.89 ± 0.03°C; P = 0.44). Core temperature 24-h profiles did not differ significantly between 11 normal-weight and 19 obese subjects (P = 0.274). Women had a mean core body temperature ≈0.23°C greater than that of men (36.99 ± 0.03°C compared with 36.76 ± 0.03°C; P < 0.0001).

CONCLUSIONS

Obesity is not generally associated with a reduced core body temperature. It may be necessary to study individuals with function-altering mutations in core temperature-regulating genes to determine whether differences in the core body temperature set point affect the regulation of human body weight. These trials were registered at clinicaltrials.gov as NCT00428987 and NCT00266500.

Adiposity and human regional body temperature

BACKGROUND

Human obesity is associated with increased heat production; however, subcutaneous adipose tissue provides an insulating layer that impedes heat loss. To maintain normothermia, therefore, obese individuals must increase their heat dissipation.

OBJECTIVE

The objective was to test the hypothesis that temperature in a heat-dissipating region of the hand is elevated in obese adults.

DESIGN

Obese [body mass index (in kg/m(2)) > or = 30] and normal-weight (NW; body mass index = 18-25) adults were studied under thermoneutral conditions at rest. Core body temperature was measured by using ingested telemetric capsules. The temperatures of the third fingernail bed of the right hand and of abdominal skin from an area 1.5 cm inferior to the umbilicus were determined by using infrared thermography. Abdominal skin temperatures were also measured via adhesive thermistors that were placed over a prominent skin-surface blood vessel and over an adjacent nonvessel location. The groups were compared by analysis of covariance with age, sex, race, and room temperature as covariates.

RESULTS

Core temperature did not differ significantly between the 23 obese and 13 NW participants (P = 0.74). However, infrared thermography-measured fingernail-bed temperature was significantly higher in obese subjects than in NW subjects (33.9 +/- 0.7 degrees C compared with 28.6 +/- 0.9 degrees C; P < 0.001). Conversely, infrared thermography-measured abdominal skin temperature was significantly lower in obese subjects than in NW subjects (31.8 +/- 0.2 degrees C compared with 32.8 +/- 0.3 degrees C; P = 0.02). Nonvessel abdominal skin temperatures measured by thermistors were also lower in obese subjects (P = 0.04).

CONCLUSIONS

Greater subcutaneous abdominal adipose tissue in obese adults may provide a significant insulating layer that blunts abdominal heat transfer. Augmented heat release from the hands may offset heat retention in areas of the body with greater adiposity, thereby helping to maintain normothermia in obesity. This trial was registered at clinicaltrials.gov as NCT00266500.