Tumor specific novel taxoid-monoclonal antibody conjugates

Stimulus-cleavable chemistry in the field of controlled drug delivery

This review comprehensively summarises stimulus-cleavable linkers from various research areas and their cleavage mechanisms, thus provides an insightful guideline to extend their potential applications to controlled drug release from nanomaterials.

Design, Synthesis, and Biological Evaluation of Novel Biotinylated Podophyllotoxin Derivatives as Potential Antitumor Agents

Podophyllotoxin has long been used as an active substance for cytotoxic activity. Fourteen novel biotinylated podophyllotoxin derivatives were designed, synthesized, and evaluated for cytotoxic activity for this study. The synthesized compounds were evaluated for cytotoxic activity in the following human cancer cell lines, SW480, MCF-7, A-549, SMMC-7721, and HL-60 by MTT assay. Most of them exhibited potent cytotoxic effects and compound 15 showed the highest cytotoxic activity among the five cancer cell lines tested, having its IC50 values in the range of 0.13 to 0.84 μM. Apoptosis analysis revealed that compound 15 caused obvious induction of cell apoptosis. Compound 15 significantly down-regulated the expression level of the marker proteins (caspase-3 and PARP) in H1299 and H1975 cells, activated the transcription of IRE1α, increased the expression of GRP78 and XBP-1s, and finally induced apoptosis of H1299 cells. In vivo studies showed that 15 at a dose of 20 mg/kg suppressed tumor growth of S180 cell xenografts in icr mice significantly. Further molecular docking studies suggested that compound 15 could bind well with the ATPase domain of Topoisomerase-II. These data suggest that compound 15 is a promising agent for cancer therapy deserving further research.

Design, Synthesis, and Biological Evaluations of Asymmetric Bow-Tie PAMAM Dendrimer-Based Conjugates for Tumor-Targeted Drug Delivery

A unique asymmetric bow-tie poly(amidoamine) (PAMAM) dendrimer (ABTD) scaffold was designed and developed as a well-defined macromolecular carrier for tumor-targeted drug delivery. The ABTD scaffold in this study consists of a G3-half-dendron (G3-HD) unit and a G1-half-dendron (G1-HD) unit, bearing thiol moiety in each unit and a bis(maleimide) linker unit, which undergo sequential thiol-maleimide coupling to assemble the scaffold. This assembly methodology is applicable to all other combinations of different generations of PAMAM dendrimers. In the prototype ABTD in this study, 16 biotin moieties were tethered to the G3-HD unit and 4 payloads (new-generation taxoid) to the G1-HD via a self-immolative linker to form an ABTD-tumor-targeting conjugate (ABTD-TTC-1). Two other ABTD-TTCs were synthesized, wherein the G1-HD unit was tethered to a fluorescence-labeled taxoid or to a fluorescent probe. These three ABTD-TTCs were constructed by using a common key ABTD 6 bearing a terminal acetylene group in the G1-HD unit, which was fully characterized as a single molecule by high-resolution mass spectrometry and NMR despite its high molecular weight (M_w: 12 876). Then, the click reaction was employed to couple ABTD 6 with a small-molecule payload or fluorescence probe unit bearing a terminal azide moiety. ABTD-TTC-3, as a surrogate of ABTD-TTC-2, showed substantially enhanced internalization into two cancer cell lines via receptor-mediated endocytosis, attributed to multibinding effect. ABTD-TTC-1 exhibited a remarkable selectivity to cancer cells (1400-7500 times) compared to human normal cells, which demonstrates the salient feature and bright prospect of the ABTD-based tumor-targeted drug-delivery system.

Preclinical development of drug delivery systems for paclitaxel-based cancer chemotherapy

Paclitaxel (PTX) is one of the most successful drugs ever used in cancer chemotherapy, acting against a variety of cancer types. Formulating PTX with Cremophor EL and ethanol (Taxol®) realized its clinical potential, but the formulation falls short of expectations due to side effects such as peripheral neuropathy, hypotension, and hypersensitivity. Abraxane®, the albumin bound PTX, represents a superior replacement of Taxol® that mitigates the side effects associated with Cremophor EL. While Abraxane® is now considered a gold standard in chemotherapy, its 21% response rate leaves much room for further improvement. The quest for safer and more effective cancer treatments has led to the development of a plethora of innovative PTX formulations, many of which are currently undergoing clinical trials. In this context, we review recent development of PTX drug delivery systems and analyze the design principles underpinning each delivery strategy. We chose several representative examples to highlight the opportunities and challenges of polymeric systems, lipid-based formulations, as well as prodrug strategies.

Biotransformation, a promising technology for anti-cancer drug development

With the high morbidity and mortality caused by cancer, finding new and more effective anti-cancer drugs is very urgent. In current research, biotransformation plays a vital role in the research and development of cancer drugs and has obtained some achievements. In this review, we have summarized four applications as follows: to exploit novel anti-cancer drugs, to improve existing anti-cancer drugs, to broaden limited anti-cancer drug resources and to investigate correlative mechanisms. Three different groups of important anti-cancer compounds were assessed to clarify the current practical applications of biotransformation in the development of anti-cancer drugs.

4-Dodecylbenzenesulfonic acid (DBSA) promoted solvent-free diversity-oriented synthesis of primary carbamates, S-thiocarbamates and ureas

A simple and efficient solvent-free preparation of primary carbamates, S-thiocarbamates and ureas from alcohols, phenols, thiols and amines in the presence of 4-dodecylbenzenesulfonic acid, as a cheap and green Brønsted acid, has been described.

Synthesis and tumor cell growth inhibitory activity of biotinylated annonaceous acetogenins

Nineteen biotinylated squamocin/bullatacin derivatives have been synthesized for targeted delivery to biotin receptor overexpressed tumor cells. Most biotinylated squamocin and bullatacin derivatives show similar in vitro cytotoxicity against the biotin receptor non-overexpressed L1210 cells as squamocin and bullatacin, respectively, while against biotin receptor overexpressed 4T1 and P815 tumor cells, several derivatives show significantly higher potency and better selectivity. Among all the synthesized compounds, 15,28-di-O-(6-biotinylamidohexanoyl)squamocin (16) is the most potent, which is 10 and 26 times more active than squamocin against 4T1 and P815 cells, respectively. Compound 16 also appears to be six and fifteen times more selective than squamocin towards 4T1 and P815 cells, respectively, against L1210 cells. The structure activity relationship analysis has revealed that the preferred site for biotinylation is different for squamocin and bullatacin, and it also depends on whether a linking spacer is present.

Biotin-conjugated tumour-targeting photocytotoxic iron(III) complexes

Iron(III) complexes [FeL(B)] (1-4) of a tetradentate phenolate-based ligand (H3L) and biotin-conjugated dipyridophenazine bases (B), viz. 7-aminodipyrido [3,2-a:2',3'-c]-phenazine (dppza in 1), (N-dipyrido[3,2-a:2',3'-c]-phenazino)amidobiotin (dppzNB in 2), dipyrido [3,2-a:2',3'-c]-phenazine-11-carboxylic acid (dppzc in 3) and 2-((2-biotinamido)ethyl) amido-dipyrido[3,2-a:2',3'-c]-phenazine (dppzCB in 4) are prepared, characterized and their interaction with streptavidin and DNA and their photocytotoxicity and cellular uptake in various cells studied. The high-spin iron(III) complexes display Fe(III)/Fe(II) redox couple near -0.7 V versus saturated calomel electrode in dimethyl sulfoxide-0.1 M tetrabutylammonium perchlorate. The complexes show non-specific interaction with DNA as determined from the binding studies. Complexes with appended biotin moiety show similar binding to streptavidin as that of free biotin, suggesting biotin conjugation to dppz does not cause any loss in its binding affinity to streptavidin. The photocytotoxicity of the complexes is tested in HepG2, HeLa and HEK293 cell lines. Complex 2 shows higher photocytotoxicity in HepG2 cells than in HeLa or HEK293, forming reactive oxygen species. This effect is attributed to the presence of overexpressed sodium-dependent multi-vitamin transporters in HepG2 cells. Microscopic studies in HepG2 cells show internalization of the biotin complexes 2 and 4 essentially occurring by receptor-mediated endocytosis, which is similar to that of native biotin and biotin fluorescein isothiocyanate conjugate.

Optimising the delivery of tubulin targeting agents through antibody conjugation

Despite their side effect profile, there currently remains a heavy reliance on traditional cytotoxics and particularly tubulin targeting agents in cancer chemotherapy. To address this concern, significant progress has been made in the selective delivery of drugs to the tumour site. This review will examine the published data in support of the hypothesis that forming antibody conjugates of tubulin targeting agents is an effective approach towards their more effective delivery to the tumour site. Particular emphasis will be placed on the diversity of concepts under investigation, the efficacy of resultant conjugates, evidence of decreased resistance and the side effect profiles of the conjugates.

Mechanism-based tumor-targeting drug delivery system. Validation of efficient vitamin receptor-mediated endocytosis and drug release

An efficient mechanism-based tumor-targeting drug delivery system, based on tumor-specific vitamin-receptor mediated endocytosis, has been developed. The tumor-targeting drug delivery system is a conjugate of a tumor-targeting molecule (biotin: vitamin H or vitamin B-7), a mechanism-based self-immolative linker and a second-generation taxoid (SB-T-1214) as the cytotoxic agent. This conjugate (1) is designed to be (i) specific to the vitamin receptors overexpressed on tumor cell surface and (ii) internalized efficiently through receptor-mediated endocytosis, followed by smooth drug release via glutathione-triggered self-immolation of the linker. In order to monitor and validate the sequence of events hypothesized, i.e., receptor-mediated endocytosis of the conjugate, drug release, and drug-binding to the target protein (microtubules), three fluorescent/fluorogenic molecular probes (2, 3, and 4) were designed and synthesized. The actual occurrence of these processes was unambiguously confirmed by means of confocal fluorescence microscopy (CFM) and flow cytometry using L1210FR leukemia cells, overexpressing biotin receptors. The molecular probe 4, bearing the taxoid linked to fluorescein, was also used to examine the cell specificity (i.e., efficacy of receptor-based cell targeting) for three cell lines, L1210FR (biotin receptors overexpressed), L1210 (biotin receptors not overexpressed), and WI38 (normal human lung fibroblast, biotin receptor negative). As anticipated, the molecular probe 4 exhibited high specificity only to L1210FR. To confirm the direct correlation between the cell-specific drug delivery and anticancer activity of the probe 4, its cytotoxicity against these three cell lines was also examined. The results clearly showed a good correlation between the two methods. In the same manner, excellent cell-specific cytotoxicity of the conjugate 1 (without fluorescein attachment to the taxoid) against the same three cell lines was confirmed. This mechanism-based tumor-targeting drug delivery system will find a range of applications.

Prospects and challenges of the development of lipoprotein-based formulations for anti-cancer drugs

This review evaluates drug delivery systems that involve intact plasma lipoproteins or some of their components. These complex macromolecules transport highly water-insoluble compounds (cholesteryl esters and triacylglycerols) in their natural environment - a property that renders them ideal carriers of hydrophobic drugs. Particular emphasis is placed on the application of lipoproteins as drug delivery agents in cancer chemotherapy. The history and present activity regarding lipoprotein-based formulations are reviewed, with the primary focus on the smaller sized (low and high density) lipoprotein-based formulations and their potential clinical and commercial value. The use of both native and synthetic lipoproteins as drug delivery agents are discussed from the standpoint of therapeutic efficacy, as well as commercial feasibility. The advantages of lipoprotein-based drug delivery formulations are compared with other drug delivery models, with the primary focus on liposomal preparations. Finally, an expert opinion is provided, regarding the potential use of lipoprotein-based formulations in cancer treatment, taking into consideration the major advantages (biocompatibility, safety, drug solubility) and the barriers (manufacturing protein components, financial interest, investments) to their commercial development.

Guided molecular missiles for tumor-targeting chemotherapy--case studies using the second-generation taxoids as warheads

A long-standing problem in cancer chemotherapy is the lack of tumor-specific treatments. Traditional chemotherapy relies on the premise that rapidly proliferating cancer cells are more likely to be killed by a cytotoxic agent. In reality, however, cytotoxic agents have very little or no specificity, which leads to systemic toxicity, causing undesirable severe side effects. Therefore, the development of innovative and efficacious tumor-specific drug delivery protocols or systems is urgently needed. A rapidly growing tumor requires various nutrients and vitamins. Thus, tumor cells overexpress many tumor-specific receptors, which can be used as targets to deliver cytotoxic agents into tumors. This Account presents our research program on the discovery and development of novel and efficient drug delivery systems, possessing tumor-targeting ability and efficacy against various cancer types, especially multidrug-resistant tumors. In general, a tumor-targeting drug delivery system consists of a tumor recognition moiety and a cytotoxic warhead connected directly or through a suitable linker to form a conjugate. The conjugate, which can be regarded as a "guided molecular missile", should be systemically nontoxic, that is, the linker must be stable in blood circulation, but upon internalization into the cancer cell, the conjugate should be readily cleaved to regenerate the active cytotoxic warhead. These novel "guided molecular missiles" are conjugates of the highly potent second-generation taxoid anticancer agents with tumor-targeting molecules through mechanism-based cleavable linkers. These conjugates are specifically delivered to tumors and internalized into tumor cells, and the potent taxoid anticancer agents are released from the linker into the cytoplasm. We have successfully used omega-3 polyunsaturated fatty acids, in particular DHA, and monoclonal antibodies (for EGFR) as tumor-targeting molecules for the conjugates, which exhibited remarkable efficacy against human tumor xenografts in animal models. We have developed self-immolative disulfide linkers wherein the glutathione-triggered cascade drug release takes place to generate the original anticancer agent. The use of disulfide linkers is attractive beacuse it takes into account the fact that the concentration of glutathione is much higher (>1000 times) in tumor cells than in blood plasma. In order to monitor and elucidate the mechanism of tumor-targeting, internalization, and drug release, several fluorescent and fluorogenic probes using biotin as the tumor-targeting module were developed and used. Then, the progressive occurrence of the designed receptor-mediated endocytosis, drug release, and drug binding to the target protein (microtubules) has been successfully observed and confirmed by means of confocal fluorescence microscopy. These "guided molecular missiles" provide bright prospects for the development of highly efficacious new generation drugs for cancer chemotherapy.

Cell killing by antibody-drug conjugates

Antibody-drug conjugates (ADCs) are designed to specifically bind to and kill cells expressing their target antigens. In addition to the obvious requirement of the presence of the target antigen on the cell surface, several other factors contribute to the sensitivity of target cells to the action of ADCs. These include (i) the rate of internalization of the ADC, (ii) its proteolytic degradation in late endosomes and lysosomes and the subsequent release of cytotoxic drug, and (iii) the intracellular concentration of the released drug. In addition to killing antigen-expressing cells, some ADCs were found to kill bystander cells irrespective of their antigen expression. This review summarizes the current knowledge of the mechanisms of killing of antigen-expressing and bystander cells by antibody-drug conjugates.

Essential properties of drug-targeting delivery systems

How, if at all, can drug delivery help to create ideal drugs? After four decades of trying, an effective site-specific drug-delivery system has not yet been developed. This review draws attention to the pharmacokinetic conditions that must be met to achieve a successful performance by site-selective drug-carrier delivery systems. In a drug-carrier approach, a drug is attached to a macromolecular carrier via a chemically labile linker. The carrier transports the drug to its site of action and releases it at the target site. For this simple approach to work, several fundamental conditions (nonspecific interactions, target site access, drug release and drug suitability) must be satisfied. The importance of these essential requirements, not always recognized in the development of drug-delivery systems, is discussed and illustrated by recent examples selected from the literature.

Antibody–cytotoxic agent conjugates for cancer therapy

Antibody-based delivery of cytotoxic agents, including toxins, to tumours can dramatically reduce systemic toxicity and increase therapeutic efficacy. The advantage of a monoclonal antibody (mAb) is superior selectivity towards antigens expressed on the surface of cancer cells. Recent advances in biotechnology accelerated progress in the pharmaceutical applications of mAbs. A cytotoxic warhead is attached to a mAb in an immunoconjugate via a linker, which is stable in circulation but efficiently cleaved in the tumour tissue. The warhead, mAb and linker play important roles in the successful design of potent and efficient immunoconjugates. To date, one mAb-cytotoxic agent conjugate has been approved by the FDA and several other candidates are in various stages of clinical trials. This review describes the recent progress in the design and development of mAb-based immunoconjugates of cytotoxic agents, and summarises the criteria for the critical choices of a suitable mAb, linker and cytotoxic agent to design an efficacious immunoconjugate.