Journal: Bioconjugate chemistry
Cycloxygenase-2 (COX-2) is an attractive target for molecular imaging because it is an inducible enzyme that is expressed in response to inflammatory and proliferative stimuli. Recently, we reported that conjugation of indomethacin with carboxy-X-rhodamine dyes results in the formation of effective, targeted, optical imaging agents able to detect COX-2 in inflammatory tissues and pre-malignant and malignant tumors (Uddin et al. Cancer Res. 2010, 70, 3618-3627). The present paper summarizes the details of the structure-activity relationship (SAR) studies performed for lead optimization of these dyes. A wide range of fluorescent conjugates were designed and synthesized, and each of them was tested for their ability to selectively inhibit COX-2 as the purified protein and in human cancer cells. The SAR study revealed that indomethacin conjugates are the best COX-2-targeted agents compared to the other carboxylic acid-containing non-steroidal anti-inflammatory drugs (NSAIDs) or COX-2-selective inhibitors (COXIBs). An n-butyldiamide linker is optimal for tethering bulky fluorescent functionalities onto the NSAID or COXIB cores. The activity of conjugates also depends on the size, shape, and electronic properties of the organic fluorophores. These reagents are taken up by COX-2-expressing cells in culture, and the uptake is blocked by pretreatment with a COX inhibitor. In in vivo settings, these reagents become highly enriched in COX-2-expressing tumors compared to surrounding normal tissue, and they accumulate selectively in COX-2-expressing tumors as compared with COX-2-negative tumors implanted in the same mice. Thus, COX-2-targeted fluorescent inhibitors are useful for preclinical and clinical detection of lesions containing elevated levels of COX-2.
Liposomes are nanocarriers that deliver the payloads at the target site, leading to therapeutic drug concentrations at the diseased site and reduced toxic effects in healthy tissues. Several approaches have been used to enhance the ability of the nanocarrier to target the specific tissues, including ligand-targeted liposomes and stimuli-responsive liposomes. Ligand-targeted liposomes exhibit higher uptake by the target tissue due to the targeting ligand attached to the surface, while, the stimuli-responsive liposomes do not release their cargo unless they expose to an endogenous or exogenous stimulant at the target site. In this review, we mainly focus on the liposomes that are responsive to pathologically increased levels of enzymes at the target site. Enzyme-responsive liposomes release their cargo upon contact with the enzyme through several destabilization mechanisms: a) structural perturbation in the lipid bilayer, b) removal of a shielding polymer from the surface and increased cellular uptake, c) cleavage of a lipopeptide or lipopolymer incorporated in the bilayer, and d) activation of a prodrug in the liposomes.
Pyridoxal-5'-phosphate (PLP) represents an active form of Vitamin B6 that shows relatively fast imine formation with hydrazines under physiological conditions without the need of a catalyst. A convenient phosphate/amine conjugation protocol was developed to covalently link PLP to proteins, affording proteins capable of hydrazone formation with bioorthogonal hydrazinyl functional groups. Thus, the lectin Concanavalin A (Con A) was labeled with PLP. Pretreatment with fluorescein hydrazide gave dye-labeled Con A that labeled cell surfaces efficiently. Alternatively, pretargeting was achieved by labeling cells with Con A-PLP, then treatment in vitro with Alexa Fluor 488 hydrazide.
The rational design of molecules with selective intracellular targeting is a great challenge of contemporary chemistry and life sciences. Here, we demonstrate rational approach to development of compartment-specific fluorescent dyes from γ-aryl substituted pentamethine family. These novel dyes exhibit an extraordinary affinity and selectivity for cardiolipin in inner mitochondrial membrane and possess excellent photostability, fluorescent properties and low phototoxicity. Selective imaging of live and fixed mitochondria was achieved in various cell lines using nanomolar concentrations of these dyes. Their high localization specificity and low toxicity enables to study morphological changes, structural complexity and dynamics of mitochondria playing pivotal role in many pathological diseases. These far-red emitting dyes could also serve for a variety of biomedical applications.
Acetylcholinesterase (AChE) is the physiological target of organophosphorus nerve agent compounds. Currently, the development of a formulation for prophylactic administration of cholinesterases as bioscavengers in established risk situations of exposure to nerve agents is the incentive for many efforts. While cholinesterases bioscavengers were found to be highly effective in conferring protection against nerve agent exposure in animal models, their therapeutic use is complicated by short circulatory residence time. To create a bioscavenger with prolonged plasma half-life, compatible with biotechnological production and purification, a chimeric recombinant molecule of HuAChE coupled to the Fc region of human IgG1 was designed. The novel fusion protein, expressed in cultured cells under optimized conditions, maintains its full enzymatic activity, at levels similar to those of the native AChE enzyme. Thus, this novel fusion product retained its bioscavenging reactivity toward the organophosphate-AChE inhibitors BW284c5, propidium, soman and VX. Furthermore, when administered to mice, AChE-Fc exhibits exceptionally circulatory residence longevity (MRT of 6000 min), superior to any other known cholinesterase-based recombinant bioscavengers. Owing to its optimized pharmacokinetic performance, high reactivity toward nerve agents and ease of production, AChE-Fc emerges as a promising next-generation organophosphorus bioscavenger.
Liver X receptor (LXR) agonists have been explored as potential treatments for atherosclerosis and other diseases based on their ability to induce reverse cholesterol transport and suppress inflammation. However, this therapeutic potential has been hindered by on-target adverse effects in the liver mediated by excessive lipogenesis. Herein, we report a novel site-specific antibody-drug conjugate (ADC) that selectively delivers a LXR agonist to monocytes/macrophages while sparing hepatocytes. The unnatural amino acid para-acetylphenylalanine (pAcF) was site-specifically incorporated into anti-CD11a IgG, which binds the α-chain component of the lymphocyte function-associated antigen 1 (LFA-1) expressed on nearly all monocytes and macrophages. An aminooxy-modified LXR agonist was conjugated to anti-CD11a IgG through a stable, cathepsin B cleavable oxime linkage to afford a chemically-defined ADC. The anti-CD11a IgG-LXR agonist ADC induced LXR activation specifically in human THP-1 monocyte/macrophage cells in vitro (EC50 ~ 27 nM), but had no effect in hepatocytes, indicating that payload delivery is CD11a-mediated. Moreover, the ADC exhibited higher fold activation compared to a conventional synthetic LXR agonist T0901317 (Tularik) (3 fold). This novel ADC represents a fundamentally different strategy that uses tissue targeting to overcome the limitations of LXR agonists for potential use in treating atherosclerosis.
Activatable cell-penetrating peptides are of great interest in drug delivery because of their enhanced selectivity which can be controlled by the external stimuli that triggers their activation. The use of a specific enzymatic reaction to trigger uptake of an inert peptide offers a relevant targeting strategy because the activation process takes place in a short time and only in areas where the specific cell surface enzyme is present. To this aim, the lysine side chain of Tat peptides was modified with an enzyme-cleavable domain of minimal size. This yielded blocked Tat-peptides which were inactive but that could be activated by co-incubation with the selected enzymes.
Sacituzumab govitecan (IMMU-132) is an antibody-drug conjugate (ADC) made from a humanized anti-Trop-2 monoclonal antibody (hRS7) conjugated with the active metabolite of irinotecan, SN-38. In addition to its further characterization, as the clinical utility of IMMU-132 expands to an ever-widening range of Trop-2-expressing solid tumor types, its efficacy in new disease models needs to be explored in a non-clinical setting. Unlike most ADCs that use ultra-toxic drugs and stable linkers, IMMU-132 uses a moderately toxic drug with a moderately stable carbonate bond between SN-38 and the linker. Flow cytometry and immunohistochemistry disclosed Trop-2 is expressed in a wide range of tumor types, including gastric, pancreatic, triple-negative breast (TNBC), colonic, prostate, and lung. While cell-binding experiments reveal no significant differences between IMMU-132 and parental hRS7 antibody, surface plasmon resonance analysis using a Trop-2 CM5 chip shows a significant binding advantage for IMMU-132 over hRS7. The conjugate retained binding to the neonatal receptor, but lost greater than 60% of the antibody-dependent cell-mediated cytotoxicity activity compared to hRS7. Exposure of tumor cells to either free SN-38 or IMMU-132 demonstrated the same signaling pathways, with pJNK1/2 and p21WAF1/Cip1 up-regulation followed by cleavage of caspases 9, 7, and 3, ultimately leading to poly-ADP-ribose polymerase cleavage and double-stranded DNA breaks. Pharmacokinetics of the intact ADC in mice reveals a mean residence time (MRT) of 15.4 h, while the carrier hRS7 antibody cleared at a similar rate as unconjugated antibody (MRT = ~300 h). IMMU-132 treatment of mice bearing human gastric cancer xenografts (17.5 mg/kg; twice weekly x 4 weeks) resulted in significant anti-tumor effects compared to mice treated with a non-specific control. Clinically relevant dosing schemes of IMMU-132 administered either every other week, weekly, or twice weekly in mice bearing human pancreatic or gastric cancer xenografts demonstrate similar, significant anti-tumor effects in both models. Current Phase I/II clinical trials (ClinicalTrials.gov, NCT01631552) confirm anticancer activity of IMMU-132 in cancers expressing Trop-2, including gastric and pancreatic cancer patients.
Glypican-3 (GPC3) is a key member of the glypican family that is expressed on the cell surface by a glycosyl-phosphatidyl-inositol (GPI) anchor. It plays a significant role in hepatocellular carcinoma (HCC) development, angiogenesis and metastasis. Most HCC overexpress GPC3, whereas little GPC3 can be detected in normal adult liver and benign liver lesions. Therefore, it is important to understand the function of GPC3 in HCC tumor development as the GPC3 ligand may facilitate detection of HCC. In this study, a 12-mer peptide with the sequence of DHLASLWWGTEL (denoted as TJ12P1) was identified by screening a phage display peptide library that demonstrated ideal GPC3 binding affinity. We used TJ12P1 conjugated with near-infrared fluorescent (NIFR) dye Cy5.5 for tumor imaging. After intravenous injection of the imaging agent, TJ12P1, xenografts of high GPC3 expressing hepatocellular carcinoma cell line, HepG2, demonstrated significantly higher tumor accumulation (Tumor/Muscle ratio: 3.98 ± 0.36) than those of low GPC3 expressing prostate cancer cell line, PC3 (Tumor/Muscle ratio: 2.03 ± 0.23). More importantly, GPC3 expression in tumor samples of patients could be visualized using TJ12P1, suggesting the potential use of this peptide as a probe for HCC detection. Our study has successfully identified a promising GPC3-binding peptide ligand for detecting the GPC3 expression in HCC not only in vitro but also in vivo by its non-invasive imaging.
Methicillin-resistant Staphylococcus aureus (MRSA) have developed resistance to antibiotics of last resort such as vancomycin, linezolid and daptomycin. Additionally, their biofilm forming capability has set an alarming situation in the treatment of bacterial infections. Herein we report the potency of fatty acid comprising lysine conjugates as novel anti-MRSA agents, which were not only capable of killing growing planktonic MRSA at low concentration (MIC = 3.1-6.3 µg/mL), but also displayed potent activity against non-dividing stationary phase cells. Further, the conjugates eradicated established biofilms of MRSA. The bactericidal activity of D-lysine conjugated tetradecanoyl conjugate (D-LANA-14) is attributed to its membrane disruption against these metabolically distinct cells. In a mouse model of superficial skin-infection D-LANA-14 displayed potent in-vivo anti-MRSA activity (3 and 4 Log reduction at 20 mg/kg and 40 mg/kg respectively) without showing any skin-toxicity even at 200 mg/kg of the compound exposure. Additionally, MRSA could not develop resistance against D-LANA-14 even after 18 subsequent passages, whereas the topical anti-MRSA antibiotic fusidic acid succumbed to rapid resistance development. Collectively, the results suggested that this new class of membrane targeting conjugates bears immense potential to treat MRSA infections over conventional antibiotic therapy.