1. Controlled Release of Avastin® from the Tethadur™ Biodegradable Matrix
C.A. Kelly, D.K. Nadarassan, H. O’Brien, C. Storey, K. Webb, Q. Shabir, A. Loni and L.T. Canham pSiMedica Ltd, Malvern Hills Science Park, Malvern, Worcestershire, WR14 3SZ, UK
Introduction
Bevacizumab (Avastin®) is used to treat back of the eye diseases such as age-related macular degeneration (AMD)
Currently administered intravitreally every 1-2 months1 – T1/2 4.9 days2
Tethadur™ is a silicon-based, biocompatible, tuneable mesoporous matrix
Tethadur™ degrades over time into benign silicic acid, which occurs naturally within the body3
Therapeutics can be loaded into Tethadur™ through electrostatic attraction – silicon matrix has a negative charge above pH 4; Avastin® has a positive charge below its pI of 8.3
Sustained release can be obtained from the Tethadur™ matrix
Activity of the released Avastin® was greater than 80 % for the first time points
Tethadur™ matrix erodes into silicic acid at a linear rate (Figure 1) – projected complete erosion after 127 days
Dissolution rate of Avastin® from Tethadur™ was initially 2.4 % / time point (Table 1)
Rate slows to 0.8 % / time point after 35 times points which is comparable to the rate of Tethadur™ erosion
Model
Avastin® dissolution
Tethadur™erosion
Whole curve
T1 - 12
T34 -50
T2 – 50
Higuchi (R2) - diffusion
0.995
0.988
Hixson-Crowell (R2) - erosion
0.996
0.989
Korsmeyer-Peppas (diffusion exponent)
0.752
0.810
1.010
Rate (%/time point)
2.398
0.803
0.788
Aims
To load Avastin® into Tethadur™ and analyse the release and activity by VEGF-binding
To monitor the degradation of Tethadur™ into silicic acid
To use dissolution models to study Avastin® release from the Tethadur™ matrix
Methods
Avastin® (100 µL of 25 mg/mL) was added to Tethadur™ (5 mg) and left at room temperature overnight (n=2)
Vitreous-like media (900 µL) was added and the samples centrifuged at rpm for 5 minutes. Supernatant was removed and analysed by SE-HPLC to calculate the Avastin® loading
Fresh vitreous-like media (1 mL) was added and the release at 37 °C monitored daily via 50 % media replacement. Protein release was measured by SE-HPLC
VEGF-binding activity of released Avastin® was monitored by ELISA
Tethadur™ (10 mg) erosion in vitreous-like media (2 mL) at 37 °C was monitored daily via a 50 % media replacement. Silicon content in samples was analysed by inductively coupled plasma optical emission spectroscopy (ICP-OES)
Changes in physical properties of Tethadur™ during erosion were monitored using sacrificial samples (200 mg Tethadur™ in 40 mL vitreous-like media with a daily 50% media replacement) by TGA, BET and particle sizing
Higuchi, Hixson-Crowell and Korsmeyer-Peppas models were used to analyse the methods of Avastin® release
Table 1: Modelling analysis of the release of Avastin® from Tethadur™
Linear regression analysis of the Higuchi and Hixson-Crowell models show release by both diffusion from and erosion of the matrix
Korsmeyer-Peppas model shows initial release was via both diffusion and erosion followed by erosion only after 35 time points
Matrix erosion results in a % decrease in the D50 particle size
Figure 2: Effect of matrix erosion on Tethadur™ particle size (D50)
Results
Avastin® was successfully loaded (24.2 ± 0.3 %w/w) into the Tethadur™ matrix
Controlled release of Avastin® from Tethadur™ was maintained for over 50 time points (Figure 1) with only a 12 % burst – approximate release 23 µg/day
Pore volume and surface area decrease during matrix erosion by 22.9 and 6.9 %, respectively
Pore size increases by 4.5 %
Figure 3: Effect of matrix erosion on the physical properties of Tethadur™
Conclusion
Avastin® was loaded onto Tethadur™ with a high payload – 24 %w/w
Low burst and controlled release for over 50 time points
Activity of released Avastin® high (>80 %) for over 30 time points
Erosion of the matrix into benign silicic acid results in a decrease in particle size, surface area and pore volume coupled with an increase in pore diameter
Figure 1: Dissolution of Avastin® from Tethadur™ and erosion of the matrix
References
Tuffail, A.; et al. BMJ  :2459
2. Moisseiev, E.; et al. Graefes Arch Clin Exp Ophthalmol (2):331-7
3. Nieto, A.; Hou, H; Sailor, M.J.; Freeman, W.R.; Cheng, L. Exp Eye Res :161-8