Scientist at work in a laboratory

Presented as a remarkable technological revolution capable of delivering novel diagnostics, treatments for unmanageable diseases, and opportunities for tissue repair, the promise of rationally designed of nanoparticle-based drugs began almost half a century ago. Today, only a limited number of both diagnostic and therapeutic agents have completed the complex journey from lab to routine clinical use.

When treating patients with cancer and hematological malignancies, the delivery of anticancer drugs has often been hindered by drawbacks related to poor solubility and poor pharmacokinetics, leading to severe adverse side effects and multidrug resistance in patients. Nanoparticle-based drugs (nanosized therapeutics and imaging agents) were developed as the ‘revolution’ to palliate these problems by improving drug delivery, ultimately opening the era of nanomedicine in oncology.

Nanoparticle-based drug delivery systems include a variety of cytotoxic-carrying agents, including liposomes, antibody-drug conjugates (ADCs), carbon nanotubes, dendrimers, polymeric micelles, polymeric conjugates, and polymeric nanoparticles.  These agents may be both passive or active targeted therapeutics designed to enhance the permeability, retention or the functionalization of the surface of the carriers.

Among these agents, liposomal agents have been by far the most used for drug delivery, with liposomal doxorubicin (Doxil®) receiving approval by the U.S. Food and Drug Administration as early as 1995.

Antibody-drug conjugates (ADC) have also been touted as effective, targeted drugs. And with seven approved drugs on the market, ADCs have become a powerful class of therapeutic agents in oncology and hematology.

Other promising nanoparticle-based drug delivery systems are currently undergoing advanced clinical trials or have received approval for clinical applications.

However, despite attractive results observed in preclinical studies, many well-designed nanoparticle-based drugs fell short of expectations when tested in patients, evidencing the gap between design and clinical translation.

Passive or Active delivery
For decades researchers have debated whether nanoparticles can be best delivered to tumors passively, allowing the nanoparticles to diffuse into tumors and become held in place, or actively, adding a targeted antibody to bind to specific cancer cell receptors.

This debate has let to a reevaluation of how nanoparticle-based (therapeutic) anticancer-drugs can be engineered to selectively detect and destroy cancer cells in solid tumors. However, relatively little analysis of nanoparticle fate and intratumor accumulation across biological models and immune cell or tumor compartments has been completed, particularly with histology or flow cytometry. [1][2][3]

Image 1.0: Histology image of HER2+ tumor showing accumulation of Herceptin-labeled nanoparticle (upper right, and blue in histology) accumulation in the tumor microenvironment (immune) and not on HER2+ cancer cells. Photo Courtesy: Robert Ivkov, Ph.D.

In a new study on human and mouse tumors in mice by researchers at the Johns Hopkins Kimmel Cancer Center, supported by the Jayne Koskinas Ted Giovanis Foundation for Health and Policy, and grants from the National Institutes of Health and the National Institutes of Health/National Cancer Institute, investigated whether labeling with a cancer-specific antibody ligand (i.e. active targeting) would be superior to its unlabeled counterpart or passive targeting.

The outcome of their research suggests that the question is not easily answered – and that the answer may even be more complicated, going beyond passive of active delivery of nanoparticle-based drugs.

Different models
The researchers tested both methods in six models of breast cancer, including five human cancer cell lines and one mouse cancer in mice. They observed that nanoparticles-based drugs coated with trastuzumab (Herceptin®; Genentech/Roche), an antibody that targets human epidermal growth factor receptor 2 (HER2)-positive breast cancer cells, were better retained in the tumors than plain nanoparticles, even in tumors that did not express the pro-growth HER2 protein.

However, they also noted that immune cells of the host exposed to nanoparticles induced an anti-cancer immune response by activating T-cells that invaded and slowed tumor growth.

A description of the work will be published on March 25, 2020 edition of in Science Advances. [4]

“It’s been known for a long time that nanoparticles, when injected into the bloodstream, are picked up by scavenger-like macrophages and other immune system cells,” noted senior study author Robert Ivkov, Ph.D., M.Sc., associate professor of radiation oncology and molecular radiation sciences at the Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins.

“Many researchers in the field have been focused on trying to reduce interactions with immune cells because they have been trying to increase the circulation time of the nanoparticles and their retention in tumor cells. But our study demonstrates that the immune cells in the tumor collect and react to the particles in such a way to stimulate an anti-cancer response. This may hold potential for advancing beyond drug delivery toward developing cancer immunotherapies,” Ivkov further explained.

In vitro experiments
As part of their study, the researchers conducted a few in vitro experiments.

First, they applied some plain starch-coated iron oxide nanoparticles and others coated with trastuzumab to five human breast cancer cell lines. Using this approach, they noted that the amount of binding between the trastuzumab-coated nanoparticles and cells depended on how much the cancer cells expressed the oncogene HER2.

In people, HER2-positive breast cancers are among the most resistant to standard chemotherapy. Trastuzumab targets the HER2-positive tumor cells and triggers the immune system as well.

Animal models
These responses were, however, surprisingly different in animal models. In separate experiments, the researchers used the nanoparticles in two immune-deficient strains of mice engrafted with cells from five human breast cancer cell lines — two that were HER2 negative and three that were HER2-positive.

When they studied the animals’ tumors 24 hours later, they noticed that nanoparticles coated with trastuzumab were found in a concentration two to five times greater than the plain nanoparticles in all types of tumors, regardless of whether they expressed the HER2 protein. They also found that the amount of trastuzumab-coated nanoparticles was even greater (tenfold) in mice that had a fully functional immune system and were bearing mouse-derived tumors.

This led the researchers to suspect that the host animals’ immune systems were interacting strongly with the nanoparticles and playing a role in determining retention of the particles in the tumor, whether or not a drug was added.

They concluded that intratumor retention of antibody-labeled nanoparticles was determined by tumor-associated dendritic cells, neutrophils, monocytes, and macrophages and not by antibody-antigen interactions.

Additional experiments, the researchers reported, revealed that tumor-associated immune cells were responsible for collecting the nanoparticles and that mice bred with an intact immune system retained more of the trastuzumab-coated nanoparticles than mice bred without a fully functioning immune system.

Tumor microenvironment
In addition, inflammatory immune cells in the tumors’ immediate surroundings, or microenvironment, seized more of the coated nanoparticles than the plain ones. Finally, in a series of 30-day experiments, the researchers found that exposure to nanoparticles inhibited tumor growth three to five times more than controls, and increased CD8-positive cancer-killing T-cells in the tumors.

“Surprisingly,” Ivkov said, “the anti-cancer immune-activating response was equally effective with exposure to either plain or trastuzumab-coated nanoparticles. Mice with defective T cells did not show tumor growth inhibition.”

Systemic exposure
This demonstrated that systemic exposure to nanoparticles can cause a systemic host immune response that leads to anti-cancer immune stimulation, and does not require (a payload of therapeutic) nanoparticles to be inside the tumors.

“Overall, our work suggests that complex interdependencies exist between the host and tumor immune responses to nanoparticle exposure,” Ivkov explained.

“These results offer intriguing possibilities for exploring nanoparticle ‘targeting’ of the tumor immune microenvironment. They also demonstrate the exciting new potential to develop nanoparticles as platforms for cancer immune therapies,” he added.

The researchers also plan to study whether the same types of immune responses can be generated for noncancer conditions, such as infectious diseases.

References
[1] Marchal S, El Hor A, Millard M, Gillon V, Bezdetnaya L. Anticancer Drug Delivery: An Update on Clinically Applied Nanotherapeutics. Drugs. 2015;75(14):1601–1611. doi:10.1007/s40265-015-0453-3
[2] Duncan R, Gaspar R. Nanomedicine(s) under the microscope. Mol Pharm. 2011;8(6):2101–2141. doi:10.1021/mp200394t
[3] Marchal S, El Hor A, Millard M, Gillon V, Bezdetnaya L. Anticancer Drug Delivery: An Update on Clinically Applied Nanotherapeutics. Drugs. 2015;75(14):1601–1611. doi:10.1007/s40265-015-0453-3
[4] Korangath P, Barnett JD, Sharma A, Henderson ET, Stewart J, Yu SH, Kandala SK, et al.
Nanoparticle interactions with immune cells dominate tumor retention and induce T cell-mediated tumor suppression in models of breast cancer. Science Advances 25 Mar 2020 : eaay1601 [Article]