Twan Lammers

Drug targeting to tumors: principles, pitfalls and (pre-) clinical progress

Many different systems and strategies have been evaluated for drug targeting to tumors over the years. Routinely used systems include liposomes, polymers, micelles, nanoparticles and antibodies, and examples of strategies are passive drug targeting, active drug targeting to cancer cells, active drug targeting to endothelial cells and triggered drug delivery. Significant progress has been made in this area of research both at the preclinical and at the clinical level, and a number of (primarily passively tumor-targeted) nanomedicine formulations have been approved for clinical use. Significant progress has also been made with regard to better understanding the (patho-) physiological principles of drug targeting to tumors. This has led to the identification of several important pitfalls in tumor-targeted drug delivery, including I) overinterpretation of the EPR effect; II) poor tumor and tissue penetration of nanomedicines; III) misunderstanding of the potential usefulness of active drug targeting; IV) irrational formulation design, based on materials which are too complex and not broadly applicable; V) insufficient incorporation of nanomedicine formulations in clinically relevant combination regimens; VI) negligence of the notion that the highest medical need relates to metastasis, and not to solid tumor treatment; VII) insufficient integration of non-invasive imaging techniques and theranostics, which could be used to personalize nanomedicine-based therapeutic interventions; and VIII) lack of (efficacy analyses in) proper animal models, which are physiologically more relevant and more predictive for the clinical situation. These insights strongly suggest that besides making ever more nanomedicine formulations, future efforts should also address some of the conceptual drawbacks of drug targeting to tumors, and that strategies should be developed to overcome these shortcomings.

Tumour-targeted nanomedicines: principles and practice.

Drug targeting systems are nanometre-sized carrier materials designed for improving the biodistribution of systemically applied (chemo)therapeutics. Various different tumour-targeted nanomedicines have been evaluated over the years, and clear evidence is currently available for substantial improvement of the therapeutic index of anticancer agents. Here, we briefly summarise the most important targeting systems and strategies, and discuss recent advances and future directions in the development of tumour-targeted nanomedicines.

Passive versus Active Tumor Targeting Using RGD- and NGR-Modified Polymeric Nanomedicines

Enhanced permeability and retention (EPR) and the (over-) expression of angiogenesis-related surface receptors are key features of tumor blood vessels. As a consequence, EPR-mediated passive and Arg-Gly-Asp (RGD) and Asn-Gly-Arg (NGR) based active tumor targeting have received considerable attention in the last couple of years. Using several different in vivo and ex vivo optical imaging techniques, we here visualized and quantified the benefit of RGD- and NGR-based vascular vs EPR-mediated passive tumor targeting. This was done using ∼ 10 nm sized polymeric nanocarriers, which were either labeled with DY-676 (peptide-modified polymers) or with DY-750 (peptide-free polymers). Upon coinjection into mice bearing both highly leaky CT26 and poorly leaky BxPC3 tumors, it was found that vascular targeting did work, resulting in rapid and efficient early binding to tumor blood vessels, but that over time, passive targeting was significantly more efficient, leading to higher overall levels and to more efficient retention within tumors. Although this situation might be different for larger carrier materials, these insights indicate that caution should be taken not to overestimate the potential of active over passive tumor targeting.

Core-crosslinked polymeric micelles with controlled release of covalently entrapped doxorubicin

Doxorubicin (DOX) is clinically applied in cancer therapy, but its use is associated with dose limiting severe side effects. Core-crosslinked biodegradable polymeric micelles composed of poly(ethylene glycol)-b-poly[N-(2-hydroxypropyl) methacrylamide-lactate] (mPEG-b-p(HPMAm-Lac(n))) diblock copolymers have shown prolonged circulation in the blood stream upon intravenous administration and enhanced tumor accumulation through the enhanced permeation and retention (EPR) effect. However a (physically) entrapped anticancer drug (paclitaxel) was previously shown to be rapidly eliminated from the circulation, likely because the drug was insufficiently retained in the micelles. To fully exploit the EPR effect for drug targeting, a DOX methacrylamide derivative (DOX-MA) was covalently incorporated into the micellar core by free radical polymerization. The structure of the doxorubicin derivative is susceptible to pH-sensitive hydrolysis, enabling controlled release of the drug in acidic conditions (in either the intratumoral environment and/or the endosomal vesicles). 30-40% w/w of the added drug was covalently entrapped, and the micelles with covalently entrapped DOX had an average diameter of 80 nm. The entire drug payload was released within 24 h incubation at pH 5 and 37 degrees C, whereas only around 5% release was observed at pH 7.4. DOX micelles showed higher cytotoxicity in B16F10 and OVCAR-3 cells compared to DOX-MA, likely due to cellular uptake of the micelles via endocytosis and intracellular drug release in the acidic organelles. The micelles showed better anti-tumor activity than free DOX in mice bearing B16F10 melanoma carcinoma. The results presented in this paper show that mPEG-b-p(HPMAm-Lac(n)) polymeric micelles with covalently entrapped doxorubicin is a system highly promising for the targeted delivery of cytostatic agents.

Simultaneous delivery of doxorubicin and gemcitabine to tumors in vivo using prototypic polymeric drug carriers

Copolymers of N-(2-hydroxypropyl)methacrylamide (HPMA) are prototypic and well-characterized polymeric drug carriers that have been broadly implemented in the delivery of anticancer therapeutics. To demonstrate that polymers, as liposomes, can be used for simultaneously delivering multiple chemotherapeutic agents to tumors in vivo, we have synthesized and evaluated an HPMA-based polymer-drug conjugate carrying 6.4wt% of gemcitabine, 5.7wt% of doxorubicin and 1.0mol% of tyrosinamide (to allow for radiolabeling). The resulting construct, i.e. poly(HPMA-co-MA-GFLG-gemcitabine-co-MA-GFLG-doxorubicin-co-MA-TyrNH(2)), was termed P-Gem-Dox, and was shown to effectively kill cancer cells in vitro, to circulate for prolonged period of time, to localize to tumors relatively selectively, and to inhibit tumor growth. As compared to control regimens, P-Gem-Dox increased the efficacy of the combination of gemcitabine and doxorubicin without increasing its toxicity, and it more strongly inhibited angiogenesis and induced apoptosis. These findings demonstrate that passively tumor-targeted polymeric drug carriers can be used for delivering two different chemotherapeutic agents to tumors simultaneously, and they thereby set the stage for more elaborate analyses on the potential of polymer-based multi-drug targeting.

Ultrasound Microbubbles for Molecular Diagnosis, Therapy, and Theranostics

Ultrasound imaging is clinically established for routine screening examinations of breast, abdomen, neck, and other soft tissues, as well as for therapy monitoring. Microbubbles as vascular contrast agents improve the detection and characterization of cancerous lesions, inflammatory processes, and cardiovascular pathologies. Taking advantage of the excellent sensitivity and specificity of ultrasound for microbubble detection, molecular imaging can be realized by binding antibodies, peptides, and other targeting moieties to microbubble surfaces. Molecular microbubbles directed against various targets such as vascular endothelial growth factor receptor-2, vascular cell adhesion molecule 1, intercellular adhesion molecule 1, selectins, and integrins were developed and were shown in preclinical studies to be able to selectively bind to tumor blood vessels and atherosclerotic plaques. Currently, the first microbubble formulations targeted to angiogenic vessels in prostate cancers are being evaluated clinically. However, microbubbles can be used for more than diagnosis: disintegrating microbubbles emit acoustic forces that are strong enough to induce thrombolysis, and they can also be used for facilitating drug and gene delivery across biologic barriers. This review on the use of microbubbles for ultrasound-based molecular imaging, therapy, and theranostics addresses innovative concepts and identifies areas in which clinical translation is foreseeable in the near future.

Challenges and strategies in anti-cancer nanomedicine development: An industry perspective

Successfully translating anti-cancer nanomedicines from pre-clinical proof of concept to demonstration of therapeutic value in the clinic is challenging. Having made significant advances with drug delivery technologies, we must learn from other areas of oncology drug development, where patient stratification and target-driven design have improved patient outcomes. We should evolve our nanomedicine development strategies to build the patient and disease into the line of sight from the outset. The success of small molecule targeted therapies has been significantly improved by employing a specific decision-making framework, such as AstraZenecas 5R principle: right target/efficacy, right tissue/exposure, right safety, right patient, and right commercial potential. With appropriate investment and collaboration to generate a platform of evidence supporting the end clinical application, a similar framework can be established for enhancing nanomedicine translation and performance. Building informative data packages to answer these questions requires the following: (I) an improved understanding of the heterogeneity of clinical cancers and of the biological factors influencing the behaviour of nanomedicines in patient tumours; (II) a transition from formulation-driven research to disease-driven development; (III) the implementation of more relevant animal models and testing protocols; and (IV) the pre-selection of the patients most likely to respond to nanomedicine therapies. These challenges must be overcome to improve (the cost-effectiveness of) nanomedicine development and translation, and they are key to establishing superior therapies for patients.

Recent Progress in Nanomedicine: Therapeutic, Diagnostic and Theranostic Applications

In recent years, the use of nanomedicine formulations for therapeutic and diagnostic applications has increased exponentially. Many different systems and strategies have been developed for drug targeting to pathological sites, as well as for visualizing and quantifying important (patho-) physiological processes. In addition, ever more efforts have been undertaken to combine diagnostic and therapeutic properties within a single nanomedicine formulation. These so-called nanotheranostics are able to provide valuable information on drug delivery, drug release and drug efficacy, and they are considered to be highly useful for personalizing nanomedicine-based (chemo-) therapeutic interventions.

Noninvasive Imaging of Nanomedicines and Nanotheranostics: Principles, Progress, and Prospects

Noninvasive imaging is used for many different (pre)clinical purposes, ranging from disease diagnosis, disease staging, and treatment monitoring to the visualization and quantification of nanomedicine-mediated drug targeting and (triggered) drug release. Noninvasive imaging can be employed to visualize and quantify how efficient passive or active drug targeting is in individual patients and, on this basis, to preselect patients likely to respond to nanomedicine-based chemotherapeutic interventions. In addition, it can be used to visualize the off-target localization of nanomedicines, e.g., in potentially endangered healthy tissues, which under certain circumstances might lead to exclusion from targeted treatment. Moreover, by systematically integrating imaging also during follow-up and by closely monitoring therapeutic responses upon nanomedicine treatment, clinical decision making can be facilitated and improved, as decisions on whether or not to (dis)continue treatment and on whether or not to adjust drug doses can be made relatively early on. Noninvasive imaging may be particularly useful in the case of metastatic disease. By subsequently performing PET or SPECT scans with radionuclide-labeled nanomedicines, information can be obtained on the accumulation of these formulations in both primary tumors and metastases, and treatment protocols can be adapted accordingly.

Multidrug resistance: Physiological principles and nanomedical solutions

Multidrug resistance (MDR) is a pathophysiological phenomenon employed by cancer cells which limits the prolonged and effective use of chemotherapeutic agents. MDR is primarily based on the over-expression of drug efflux pumps in the cellular membrane. Prominent examples of such efflux pumps, which belong to the ATP-binding cassette (ABC) superfamily of proteins, are Pgp (P-glycoprotein) and MRP (multidrug resistance-associated protein), nowadays officially known as ABCB1 and ABCC1. Over the years, several strategies have been evaluated to overcome MDR, based not only on the use of low-molecular-weight MDR modulators, but also on the implementation of 1-100(0) nm-sized drug delivery systems. In the present manuscript, after introducing the most important physiological principles of MDR, we summarize prototypic nanomedical strategies to overcome multidrug resistance, including the use of carrier materials with intrinsic anti-MDR properties, the use of nanomedicines to modify the mode of cellular uptake, and the co-formulation of chemotherapeutic drugs together with low- and high-molecular-weight MDR inhibitors within a single drug delivery system. While certain challenges still need to be overcome before such constructs and concepts can be widely applied in the clinic, the insights obtained and the progress made strongly suggest that nanomedicine formulations hold significant potential for improving the treatment of multidrug-resistant malignancies.