Researchers from Calando Pharmaceuticals this week published data demonstrating that the company’s lead siRNA drug candidate, CALAA-01, can be systemically administered to non-human primates with no adverse effects using a proprietary nanoparticle delivery technology.
Publication of the data, which appears online in the early edition of the Proceedings of the National Academy of Sciences, comes about five months after Calando first presented some of the findings at last year’s Oligonucleotide Therapeutics Society annual meeting (see RNAi News, 10/26/2006).
But the PNAS paper also includes pharmacokinetic data that were not presented at the meeting.
"The experimental work reported in this paper shows that CALAA-01 can safely be repeatedly administered to large animals," Calando CSO Jeremy Heidel said in a statement this week. "We observed a lack of significant immunostimulation and, notably, the absence of complement activation, even at the highest dose administered.”
Based on the study results, Calando is proceeding with full investigational new drug-enabling toxicology studies “as the next step toward commencing what we believe will be the first phase I clinical trial with a targeted, systemic formulation of siRNA by the end of 2007,” CEO John Petrovich added in the statement.
A relative newcomer to the RNAi drugs field, Calando is banking on a delivery system that comprises a linear, cyclodextrin-containing polycation capable of binding to the anionic backbone of an siRNA. When mixed together, the polymer and siRNA self-assemble into nanoparticles, roughly 50 nanometers in diameter, that are protected from nuclease degradation in blood serum.
According to Calando, the cyclodextrin in the polymer allows stabilizing agents to be attached to the surface of the particles. These agents have terminal adamantane groups that form inclusion complexes with cyclodextrin and contain polyethylene glycol, which prevents aggregation while preventing degradation. Additionally, ligands to cell-surface receptors — in the case of the primate study, human transferrin — can be covalently attached to the adamantane-PEG modifier, which allows the particles to be targeted to tissues of interest.
In its lead pipeline program, Calando is using the technology to deliver an siRNA targeting the M2 subunit of ribonucleotide reductase, an enzyme that catalyzes the formation of the deoxyribonucleotide precursors required for DNA synthesis.
The RR subunit M2 has been established as a cancer target given its essential role in DNA synthesis and repair, as well as cell proliferation, and a number of companies are pursuing development of drugs against RRM2.
In the study published in PNAS and reported by RNAi News last year, Calando used its nanoparticle technology to deliver RRM2-targeting, non-modified siRNAs to three female cynomolgus monkeys in escalating doses of 3, 9, and 27 mg/kg over a 17-to 18-day period.
At the two lowest doses, the nanoparticles were well-tolerated, according to the PNAS paper.
However, “at 27 mg siRNA/kg, elevated levels of blood urea nitrogen and creatinine are observed that are indicative of kidney toxicity,” the paper’s authors noted. “Mild elevations in alanine amino transferase and aspartate transaminase at this dose level indicate that the liver is also affected to some extent.”
Additionally, “detection of increased IL-6 levels in all animals at [the 27 mg/kg] and increased IFN-gamma in one animal indicate that this high dose level produces a mild immune response.”
In evaluating the formation of antibodies against the human transferrin portion of the nanoparticle delivery agent, the Calando researchers detected lower titers of anti-Tf antibodies, but found that the response was “not associated with any manifestations of a hypersensitivity reaction upon re-administration of the targeted nanoparticle.”
The researchers also analyzed plasma concentrations of the siRNA drug at five minutes, 30 minutes, two hours, and six hours after dosing.
"The experimental work reported in this paper shows that CALAA-01 can safely be repeatedly administered to large animals. We observed a lack of significant immunostimulation and, notably, the absence of complement activation, even at the highest dose administered.”
“All three animals [exhibited] dose-dependent siRNA levels in plasma at the first time point [of five minutes] and relatively rapid clearance … most likely owing to the intended tissue targeting,” they stated in the paper.
“A comparison of the amounts of siRNA measured at 5 [minutes] after the initial and final 3 mg/kg doses showed no significant decreases between these dosing occasions, suggesting that there was no antibody-mediated acceleration of nanoparticle clearance,” they added.
“However, because of the limited number of animals and data points, a firm conclusion cannot be drawn regarding the possible influence of anti-Tf antibody formation on nanoparticle kinetics,” the researchers noted. “Further experiments that address this issue need to be performed.
“Also, it is important to note that no adverse reactions were observed in the animals from the final 3 mg/kg dose when antibodies were present,” the authors wrote. “This finding contrasts with the adverse reactions observed with repeated administration of lipid-based systems that are associated with the presence of antibodies.”
In the end, these data suggest that CALAA-01 is “well-tolerated at levels many times higher than what we would expect to be an efficacious dose in humans,” Petrovich said.
Heidel added that the study results “are promising and suggest further investigation of CALAA-01 is warranted." In line with this, Calando remains on track to meet its previously disclosed goal of beginning human testing of the drug before the end of the year.
Last year, Heidel said that the planned phase I would evaluate the drug in patients with solid tumors of multiple origins. Calando officials were not available for additional comment by press time.