Targeting the Somatostatin Receptor 2 with the Miniaturized Drug Conjugate, PEN-221: A Potent and Novel Therapeutic for the Treatment of Small Cell Lung Cancer.

Small cell lung cancer (SCLC) is an aggressive neuroendocrine carcinoma with a 95% mortality rate with no improvement to treatment in decades, and new therapies are desperately needed. PEN-221 is a miniaturized peptide-drug conjugate (∼2 kDa) designed to target SCLC via a Somatostatin Receptor 2 (SSTR2)-targeting ligand and to overcome the high proliferation rate characteristic of this disease by using the potent cytotoxic payload, DM1. SSTR2 is an ideal target for a drug conjugate, as it is overexpressed in SCLC with limited normal tissue expression. In vitro, PEN-221 treatment of SSTR2-positive cells resulted in PEN-221 internalization and receptor-dependent inhibition of cellular proliferation. In vivo, PEN-221 exhibited rapid accumulation in SSTR2-positive SCLC xenograft tumors with quick clearance from plasma. Tumor accumulation was sustained, resulting in durable pharmacodynamic changes throughout the tumor, as evidenced by increases in the mitotic marker of G2-M arrest, phosphohistone H3, and increases in the apoptotic marker, cleaved caspase-3. PEN-221 treatment resulted in significant antitumor activity, including complete regressions in SSTR2-positive SCLC xenograft mouse models. Treatment was effective using a variety of dosing schedules and at doses below the MTD, suggesting flexibility of dosing schedule and potential for a large therapeutic window in the clinic. The unique attributes of the miniaturized drug conjugate allowed for deep tumor penetration and limited plasma exposure that may enable long-term dosing, resulting in durable tumor control. Collectively, these data suggest potential for antitumor activity of PEN-221 in patients with SSTR2-positive SCLC.

Discovery of an SSTR2-Targeting Maytansinoid Conjugate (PEN-221) with Potent Activity in Vitro and in Vivo.

Somatostatin receptor 2 (SSTR2) is frequently overexpressed on several types of solid tumors, including neuroendocrine tumors and small-cell lung cancer. Peptide agonists of SSTR2 are rapidly internalized upon binding to the receptor and linking a toxic payload to an SSTR2 agonist is a potential method to kill SSTR2-expressing tumor cells. Herein, we describe our efforts towards an efficacious SSTR2-targeting cytotoxic conjugate; examination of different SSTR2-targeting ligands, conjugation sites, and payloads led to the discovery of 22 (PEN-221), a conjugate consisting of microtubule-targeting agent DM1 linked to the C-terminal side chain of Tyr3-octreotate. PEN-221 demonstrates in vitro activity which is both potent (IC50 = 10 nM) and receptor-dependent (IC50 shifts 90-fold upon receptor blockade). PEN-221 targets high levels of DM1 to SSTR2-expressing xenograft tumors, which has led to tumor regressions in several SSTR2-expressing xenograft mouse models. The safety and efficacy of PEN-221 is currently under evaluation in human clinical trials.

Tumor Selective Silencing Using an RNAi-Conjugated Polymeric Nanopharmaceutical.

Small interfering RNA (siRNA) therapeutics have potential advantages over traditional small molecule drugs such as high specificity and the ability to inhibit otherwise "undruggable" targets. However, siRNAs have short plasma half-lives in vivo, can induce a cytokine response, and show poor cellular uptake. Formulating siRNA into nanoparticles offers two advantages: enhanced siRNA stability against nuclease degradation beyond what chemical modification alone can provide; and improved site-specific delivery that takes advantage of the enhanced permeability and retention (EPR) effect. Existing delivery systems generally suffer from poor delivery to tumors. Here we describe the formation and biological activity of polymeric nanopharmaceuticals (PNPs) based on biocompatible and biodegradable poly(lactic-co-glycolic acid) (PLGA) conjugated to siRNA via an intracellular cleavable disulfide linker (PLGA-siRNA). Additionally, these PNPs contain (1) PLGA conjugated to polyethylene glycol (PEG) for enhanced pharmacokinetics of the nanocarrier; (2) a cation for complexation of siRNA and charge compensation to avoid high negative zeta potential; and (3) neutral poly(vinyl alcohol) (PVA) to stabilize the PNPs and support the PEG shell to prevent particle aggregation and protein adsorption. The biological data demonstrate that these PNPs achieve prolonged circulation, tumor accumulation that is uniform throughout the tumor, and prolonged tumor-specific knockdown. PNPs employed in this study had no effect on body weight, blood cell count, serum chemistry, or cytokine response at doses >10 times the effective dose. PNPs, therefore, constitute a promising solution for achieving durable siRNA delivery and gene silencing in tumors.

Drug release mechanism of paclitaxel from a chitosan-lipid implant system: effect of swelling, degradation and morphology.

Localized and sustained delivery of anti-cancer agents to the tumor site has great potential for the treatment of solid tumors. A chitosan-egg phosphatidylcholine (chitosan-ePC) implant system containing PLA-b-PEG/PLA nanoparticles has been developed for the delivery of paclitaxel to treat ovarian cancer. Production of volumes of ascites fluid in the peritoneal cavity is a physical manifestation of ovarian cancer. In vitro release studies of paclitaxel from the implant were conducted in various fluids including human ascites fluid. A strong correlation (r2=0.977) was found between the release of paclitaxel in ascites fluid and PBS containing lysozyme (pH 7.4) at 37 degrees C. The drug release mechanism for this system was proposed based on swelling, degradation and morphology data. In addition, in vitro release of paclitaxel was found to be a good indicator of the in vivo release profile (correlation between release rates: r2=0.965). Release of paclitaxel was found to be sustained over a four-week period following implantation of the chitosan-ePC system into the peritoneal cavity of healthy Balb/C mice. Also, the concentrations of paclitaxel in both plasma and tissues (e.g. liver, kidney and small intestine) were found to be relatively constant.

Polycaprolactone-block-poly(ethylene oxide) micelles: a nanodelivery system for 17beta-estradiol.

Various hormone replacement regimens and delivery systems have been developed; however, there is still a need for additional, easily controllable and biocompatible systems. We have developed and characterized biocompatible polycaprolactone-block-poly(ethylene oxide) (PCL-b-PEO) micelles for the delivery of 17beta-estradiol (E2) and investigated their loading and release properties using fluorescence spectroscopy. The micelles are spherical aggregates that range in size from 20 to 40 nm, as determined by both transmission electron microscopy and dynamic light scattering. A high loading efficiency for E2 of up to 96%, as well as a high drug loading capacity of up to 4000 molecules of E2 per micelle (equivalent to 190% (w/w)), is obtainable. In addition, the E2 loading and release can be controlled by modifying the block length of the polycaprolactone core and the initial estradiol concentration. The release of E2 from the micelles showed a biphasic profile under perfect sink conditions: there is an initial burst release, followed by a slow and prolonged release for up to 5 days, until complete release is achieved. The release of E2 from the micelles was shown to be diffusional, as shown by the linearity of the release as a function of the square root of time. Approximate diffusion coefficients of the order of 10(-17) cm2/s were obtained. In vitro and in vivo experiments confirmed that the biological activity of E2 was retained after preparation of the micelles. This micelle carrier could serve as a versatile and efficient nanodelivery system for steroids and other poorly water soluble drugs that require solubilizing agents for delivery.