Subcutaneous drug delivery and the role of the lymphatics

Targeting the lymphatics using dendritic polymers (dendrimers)

Dendrimers are unique biomaterials that are constructed by the stepwise addition of layers (generations) of polymer around a central core. They can be constructed with a range of molecular weights and have a polyfunctional surface that facilitates the attachment of drugs and pharmacokinetic modifiers such PEG or targeting moieties. These properties have led to considerable interest in the development of dendrimers for a range of biomedical applications. After subcutaneous administration, larger dendrimers in particular (> 8 nm), preferentially drain from the injection site into the peripheral lymphatic capillaries and therefore have potential as lymphatic imaging agents for magnetic resonance and optical fluorescence lymphangiography and as vectors for drug-targeting to lymphatic sites of disease progression. In general, lymphatic targeting of dendrimers is enhanced by increasing size although ultimately larger constructs may be incompletely absorbed from the injection site. Increasing hydrophilicity and reducing surface charge enhances drainage from subcutaneous injection sites, but the reverse is true of uptake into lymph nodes where charge and hydrophobicity promote retention. Larger hydrophilic dendrimers are also capable of extravasation from the systemic circulation, absorption into the lymphatic system and recirculation into the blood. Lymphatic recirculation may therefore be a characteristic of PEGylated dendrimers with long systemic circulation times.

Subcutaneous absorption of monoclonal antibodies: role of dose, site of injection, and injection volume on rituximab pharmacokinetics in rats

PURPOSE

To determine the effect of dose, the anatomical site of injection, and the injection volume on subcutaneous absorption of rituximab in rats and to explore absorption mechanisms using pharmacokinetic modeling.

METHODS

Rituximab serum concentrations were measured following intravenous and subcutaneous administration at the back, abdomen, and foot of rats. Several pharmacokinetic models were developed that included linear and saturable absorption, and degradation and/or protective binding at the injection site.

RESULTS

Rituximab exhibited linear kinetics following intravenous administration; however, bioavailability following subcutaneous injection was inversely related to the dose level. For the 1 mg/kg dose, bioavailability was approximately 70% at all tested injection sites, with faster absorption from the foot (T(max) = 12 h for foot vs. 4.6 days for back). Bioavailability for the 10 mg/kg dose was 44 and 31% for the abdomen and back sites and 18% for 40 mg/kg injected at the back. A pharmacokinetic model that included binding as part of the absorption mechanism successfully captured the nonlinearities in rituximab absorption.

CONCLUSION

The anatomical site of subcutaneous injection influences the rate of absorption and bioavailability of rituximab in rats. Saturable binding may be a major determinant of the nonlinear absorptive transport of monoclonal antibodies.

Bioavailability and variability of biphasic insulin mixtures

Absorption of subcutaneously administered insulin is associated with considerable variability. Some of this variability was quantitatively explained for both soluble insulin and insulin suspensions in a recent contribution to this journal (Søeborg et al., 2009). In the present article, the absorption kinetics for mixtures of insulins is described. This requires that the bioavailability of the different insulins is considered. A short review of insulin bioavailability and a description of the subcutaneous depot thus precede the presentation of possible mechanisms associated with subcutaneous insulin degradation. Soluble insulins are assumed to be degraded enzymatically in the subcutaneous tissue. Suspended insulin crystals form condensed heaps that are assumed to be degraded from their surface by invading macrophages. It is demonstrated how the shape of the heaps affects the absorption kinetics. Variations in heap formation thus explain some of the additional variability associated with suspended insulins (e.g. NPH insulins) compared to soluble insulins. The heap model also describes how increasing concentrations of suspended insulins lead to decreasing bioavailability and lower values of Cmax. Together, the findings constitute a comprehensive, quantitative description of insulin absorption after subcutaneous administration. The model considers different concentrations and doses of soluble insulin, including rapid acting insulin analogues, insulin suspensions and biphasic insulin mixtures. The results can be used in both the development of novel insulin products and in the planning of the treatment of insulin dependent diabetic patients.

Charge variants in IgG1: Isolation, characterization, in vitro binding properties and pharmacokinetics in rats

Antibody charge variants have gained considerable attention in the biotechnology industry due to their potential influence on stability and biological activity. Subtle differences in the relative proportions of charge variants are often observed during routine biomanufacture or process changes and pose a challenge to demonstrating product comparability. To gain further insights into the impact on biological activity and pharmacokinetics (PK) of monoclonal antibody (mAb) charge heterogeneity, we isolated the major charge forms of a recombinant humanized IgG1 and compared their in vitro properties and in vivo PK. The mAb starting material had a pI range of 8.7-9.1 and was composed of about 20% acidic variants, 12% basic variants, and 68% main peak. Cation exchange displacement chromatography was used to isolate the acidic, basic, and main peak fractions for animal studies. Detailed analyses were performed on the isolated fractions to identify specific chemical modification contributing to the charge differences, and were also characterized for purity and in vitro potency prior to being administered either subcutaneously (SC) or intravenously (IV) in rats. All isolated materials had similar potency and rat FcRn binding relative to the starting material. Following IV or SC administration (10 mg/kg) in rats, no difference in serum PK was observed, indicating that physiochemical modifications and pI differences among charge variants were not sufficient to result in PK changes. Thus, these results provided meaningful information for the comparative evaluation of charge-related heterogeneity of mAbs, and suggested that charge variants of IgGs do not affect the in vitro potency, FcRn binding affinity, or the PK properties in rats.

Formulation, characterization and pharmacokinetics of praziquantel-loaded hydrogenated castor oil solid lipid nanoparticles

Aim: The purpose of this study was to formulate praziquantel (PZQ)-loaded hydrogenated castor oil (HCO) solid lipid nanoparticles (SLN) to enhance the bioavailability and prolong the systemic circulation of the drug. Materials & methods: PZQ was encapsulated into HCO nanoparticles by a hot homogenization and ultrasonication method. The physicochemical characteristics of SLN were investigated by optical microscope, scanning electron microscopy and photon correlation spectroscopy. Pharmacokinetics were studied after oral, subcutaneous and intramuscular administration in mice. Results: The diameter, polydispersivity index, ζ potential, encapsulation efficiency and loading capacity of the nanoparticles were 344.0 ± 15.1 nm, 0.31 ± 0.08, -16.7 ± 0.5 mV, 62.17 ± 6.53% and 12.43 ± 1.31%, respectively. In vitro release of PZQ-loaded HCO-SLN exhibited an initial burst release followed by a sustained release. SLN increased the bioavailability of PZQ by 14.9-, 16.1- and 2.6-fold, and extended the mean residence time of the drug from 7.6, 6.6 and 8.2 to 95.9, 151.6 and 48.2 h after oral, subcutaneous and intramuscular administration, respectively. Conclusion: The PZQ-loaded HCO-SLN could be a promising formulation to enhance the pharmacological activity of PZQ.

Microneedle-based intradermal delivery enables rapid lymphatic uptake and distribution of protein drugs

PURPOSE

The purpose of this research was to examine the pharmacokinetics (PK) of drug uptake for microneedle-based intradermal (ID) delivery of several classes of protein drugs compared to standard subcutaneous (SC) administration.

METHODS

Systemic absorption kinetics of various proteins were analyzed following microneedle-based ID delivery and standard injection methods in the swine model. Comparative PK data were determined using standard non-compartmental techniques based on blood serum levels.

RESULTS

Delivery of proteins using microneedles resulted in faster systemic availability, measured via t(max,) and increased maximal drug concentration, C(max,) over SC delivery for all proteins tested. Some agents also exhibited increased bioavailability for the ID route. Imaging studies using reporter dyes showed rapid lymphatic-mediated uptake.

CONCLUSIONS

Microneedle delivery is applicable to a wide variety of protein drugs and is capable of effective parenteral administration of therapeutic drug dosages. This delivery route alters absorption kinetics via targeting a tissue bed better perfused with lymphatic and blood vessels than the SC space. Microneedle delivery may afford various advantages, including a robust method to increase the absorption rate and bioavailability of proteins that have been challenging to deliver at therapeutic levels or with physiologically relevant profiles.

Pharmacokinetics analysis of sustained release hGH biodegradable implantable tablets using a mouse model of human ovarian cancer

This paper presents the pharmacokinetic of human growth hormone (hGH) implantable tablets tested on a human ovarian cancer mouse model. In order to obtain a sustained release device which permits to administer a high dose of the hormone that keeps its integrity and stability, three different formulations of hGH-poly (d,l-lactic-co-glycolic acid) (PLGA) were elaborated by direct compression method varying hormone load, PLGA content and compactation time. In vitro studies showed that drug release was mainly controlled by hormone load. Pharmacokinetic studies were conducted by using immunodeficient female mice. Four days before the insertion of hGH implantable tablets in the peritoneal cavity, every mouse received 5x10(6) human ovarian cancer cells (SKOV3.ip1). Hormone serum levels were monitored through bleeding from eye orbital vessels. The population pharmacokinetic model used was based on the in series tank model and model parameters were estimated using the maximum likelihood method. The null hypothesis test about differences between formulations leads us to the conclusion that the three formulations showed the same kinetic behavior except for the hGH load. The hormone release was extended all over 2 weeks but no increase or decrease in survival time was observed. These results suggest that hGH serum levels do not facilitate tumoral cells proliferation, an expected effect of hGH and this could explain why survival times of mice treated with implantable tablets are not shorter than those treated with the control ones.

PEGylation of polylysine dendrimers improves absorption and lymphatic targeting following SC administration in rats

Polylysine dendrimers have potential as highly flexible, biodegradable nanoparticular carriers that may also promote lymphatic transport. The current study was undertaken to determine the impact of PEGylation on the absorption and lymphatic transport of polylysine dendrimers modified by surface derivatisation with PEG (200, 570 or 2000Da) or 4-benzene sulphonate following SC or IV dosing. PEGylation led to the PEG(200) derived dendrimer being rapidly and completely absorbed into the blood after SC administration, however only 3% of the administered dose was recovered in pooled thoracic lymph over 30h. Increasing the PEG chain length led to a systematic decrease in absorption into the blood and an enhancement of the proportion recovered in the lymphatics (up to 29% over 30h). For the PEG(570) and PEG(2000) derived dendrimers, indirect access to the lymph via equilibration across the capillary beds also appeared to play a role in lymphatic targeting after both IV and SC dosing. In contrast, the anionic benzene sulphonate-capped dendrimer was not well absorbed from the SC injection site (26% bioavailability) into either the blood or the lymph. The data suggest that PEGylated poly-L-lysine dendrimers are well absorbed from SC injection sites and that the extent of lymphatic transport may be enhanced by increasing the size of the PEGylated dendrimer complex.

Transscleral iontophoretic and intravitreal delivery of a macromolecule: study of ocular distribution in vivo and postmortem with MRI

The distribution and clearance of macromolecules in ocular delivery are not well understood. It has been hypothesized that iontophoresis can enhance transscleral delivery of macromolecules. The objective of this study was to investigate the ocular distribution of a macromolecule after transscleral iontophoretic delivery and intravitreal injection in vivo using nuclear magnetic resonance imaging (MRI) and to compare these results. Experiments of constant current transscleral iontophoresis of 4mA or intravitreal injection were performed on New Zealand white rabbits in vivo. Iontophoresis experiments were also performed on rabbits postmortem. Galbumin (Gd-labeled albumin) was the model permeant surrogate to clinical therapeutic agents. MRI was used to monitor the distribution of the molecule in the eye after ocular iontophoresis and intravitreal injection. In addition, the conjunctiva, sclera, choroid, and retina were extracted in the transscleral iontophoresis study to determine the amounts of Galbumin in these tissues using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The results show that iontophoresis enhanced the ocular delivery of Galbumin. The macromolecule was mainly delivered into the conjunctiva and sclera in microgram quantities and then diffused towards the posterior section in the upper hemisphere of the eye in vivo. Both in vivo and postmortem studies show that the iontophoretic delivery of Galbumin into the vitreous was below the detection limit. In the intravitreal injection study, the diffusion coefficient of Galbumin in the vitreous humor was estimated to be close to that of free aqueous diffusion.

The role of the lymphatic system in subcutaneous absorption of macromolecules in the rat model

The purpose of this study was to assess the contribution of lymphatics to the systemic bioavailability of macromolecules following SC administration in a rat model. The rat model included continuous lymph collection from the thoracic lymph duct and concurrent serial blood sampling from freely moving animals. A thoracic lymph duct-jugular vein shunt produced by an implanted connective cannula, and maintained during the recovery period, enabled superior rat survival and prevented lymphatic cannula occlusion. The SC absorption of three macromolecules (bovine insulin, bovine serum albumin, and recombinant human erythropoietin alpha) was assessed in comparison to the non-lymph cannulated control group. For all tested molecules, only minimal amounts (less than 3%) of the SC administered dose were detected in the collected lymph. In the rat model, following SC administration, the macromolecules were absorbed mainly through the blood capillaries with minimal contribution of the lymphatic system to systemic bioavailability. The relatively small elevation in the lymphatic concentration, which occurred in all molecules, may be attributed to the redistribution of the molecules from the blood to the interstitial fluid compartment. These findings are important since rodents are commonly used in preclinical evaluation of macromolecular drugs.

Drug elimination kinetics following subconjunctival injection using dynamic contrast-enhanced magnetic resonance imaging

PURPOSE

To determine the elimination rates of subconjunctivally injected model drugs using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI).

METHODS

Gadolinium-diethylenetriaminopentaacetic acid (Gd-DTPA) and gadolinium-albumin (Gd-albumin) were injected in rabbits. Experiments were performed in vivo and post mortem and injection volumes of 200 and 600 microl were administered. Signal intensity values from MR images were converted to concentration of contrast agent to determine the mass clearance rates from subconjunctival space.

RESULTS

Injection volume did not have a significant effect on clearance rate for both Gd-DTPA and Gd-albumin. The clearance rate of Gd-DTPA in vivo was about nine times faster than that post mortem. The in vivo and post mortem clearance rates of Gd-albumin were not significantly different. The in vivo half-life of Gd-DTPA was about 22 min while that of Gd-albumin was about 5.3 h.

CONCLUSIONS

DCE-MRI was used to quantitatively compare the subconjunctival clearance rates of Gd-DTPA and Gd-albumin. Dynamic clearance mechanisms present in vivo significantly reduced the subconjunctival concentration of Gd-DTPA but not Gd-albumin. Lymphatic clearance does not seem to be as significant as clearance by blood, as evidenced by data from Gd-albumin injections. Larger injection volumes may allow for longer retention times and prolonged release of drug.

The absorption of darbepoetin alfa occurs predominantly via the lymphatics following subcutaneous administration to sheep

PURPOSE

To determine the contribution of the lymphatics to the systemic availability of darbepoetin alfa (DA) using an established sheep model.

MATERIALS AND METHODS

DA was administered either by intravenous (IV) injection (0.2, 0.5 or 2 microg/kg) or by subcutaneous (SC) administration (2 microg/kg) into the interdigital space of the hind leg. A SC control group was used to determine the absolute bioavailability (F (sys)). Cannulation of the peripheral lymphatics in a parallel SC group allowed the continuous collection of lymph draining the injection site and determination of the cumulative amount of DA absorbed via the lymphatics. Serum and lymph concentrations of DA were determined by ELISA. The fraction of the dose absorbed into the lymphatics (F (lymph)) relative to the fraction absorbed directly into the blood (F (blood)) was determined using a compartmental approach.

RESULTS

Dose-linear pharmacokinetics was observed within the dose range investigated. The bioavailability was virtually complete following SC injection into the interdigital space (88.4 +/- 15.7%). A high proportion of the administered dose was recovered in peripheral lymph (90.2 +/- 4.4%) resulting in a substantial reduction in the systemic availability in lymph cannulated animals (3.7%).

CONCLUSION

The high recovery of DA in the peripheral lymph demonstrated near complete absorption of this recombinant protein via the lymphatics in a lymph cannulated sheep model.