Rat PBPK model structure
A whole-body mechanistic physiologically based pharmacokinetic (m-PBPK) rat model was designed in Simbiology v. 9.6.0 (MATLAB R2019a) and broken down in 16 compartments: blood, liver, spleen, kidneys, lungs, heart, brain, adipose tissue, bones, intestines, muscle, pancreas, skin, stomach, lymph central and remaining anatomical body features. The whole m-PBPK was also arranged in 2 blocks, the first one representing vascular and interstitial spaces for the nanoparticle, while for the released drug (doxorubicin) each organ was represented by the second block (Fig. 1). The physiological parameters organ volume and blood flow were described as previously by Peters [10], and the fraction of vascular and interstitial space of each organ were described as by Shah and Betts [11]. The weight of organs was implemented as reported by the US Environmental Protection Agency [15].
Human PBPK model structure
A comparable 16 compartment structure model described for rats was used in the adult model. Physiological characteristics such as age, weight, and body mass index (BMI) were implemented as reported by the Centers for Disease Control and Prevention [13]. Organ and tissue volumes were calculated through weight anthropometric equations as described by Bosgra et al. [14]. Organ and tissue blood flows were calculated as a percentage of the cardiac output described by the US Environmental Protection Agency [15]. The fraction of vascular and interstitial space of each organ were calculated according to Gill et al. [12].
Distribution
The biodistribution of NP and released drug are represented differently [16] (Fig. 1). For the NP biodistribution two main processes were involved: (1) permeability of the NP across vascular wall into the interstitial compartment, (2) macrophage uptake of the NP into the tissue. The permeation of the NPs from the vascular space to the interstitial space considered both perfusion- and diffusion-limited approaches. Organs were grouped according to their capillary type as described before [17, 18] and adapted to the model here presented and summarised in the Table 1.
The distribution of the released drug (doxorubicin) into organs and tissues was simulated using previously published equations, and tissue-to-plasma ratios (TP) were calculated according to Poulin and Theil’s equations [19]. The tissue composition (fractional tissue volume content of phospholipid, neutral lipid, and water) of each organ for rats and human was used as described previously by Peters [10].
Table 1
Transcapillary permeability according to specific organs.
Blood capillary type | Organs | Transcapillary permeability |
CT1 (non-sinusoidal non-fenestrated) | Heart, lungs, brain, bones, muscle, adipose, skin, stomach, remaining | 0.0015 |
CT2 (non-sinusoidal fenestrated) | Intestines, pancreas, kidneys | 0.012 |
CT3 (sinusoidal with pores larger) | Liver, spleen | 0.024 |
CT4 (myeloid bone marrow sinusoidal) | Bone marrow | 0.216 |
Adapted from Sarin et al. [17] and Bachler et al. [18]. |
Doxil® availability for each tissue vascular compartment was represented through the arterial organ blood flow (Qorgan, mL/h and L/h for rat and human models, respectively) and the venous organ blood flow (organ blood flow reduced by organ lymphatic flow, Qorgan- QL,organ). The QL,organ was collected into a single compartment (lymph central) and was considered 500 times lower than the corresponding Qorgan as previously described[11]. It was assumed that lymph node fluid reabsorption is negligible, then the total lymph flow returning to the vein compartment was the sum of all organs lymph flow to keep a fluid balance [12]. Lymphatic flow from the spleen and bones tissues were considered negligible. A lymphatic reflection coefficient (σlymph) of 0.2 was set up according to previously published literature [11].
The amount of NP transported across the vascular wall was calculated considering the organ blood flow (Qorgan) normalized to the total body blood volume (Vblood) multiplied by the organ transcapillary permeability (CT):
NPinflux (mg/h) = \(\frac{Qorgan}{Vblood} \times CT\times NP\times fup\), Eq. 1
NPefflux (mg/h) = \(\frac{Qorgan}{Vblood} \times CT\times NP\times fut\), Eq. 2
where NP is expressed as the mass of nanoparticles in the vascular or in the interstitial space in milligrams, and fup and fut corresponds to the fraction unbound of the NP in the vascular space and in the tissue. It was assumed that there is no intracellular uptake of Doxil® or insignificant, once it is present mainly on vascular space, surrounding the blood vessels, and infiltrated into the extracellular space [20].
Macrophage uptake
Macrophage uptake of the NP from vascular to the interstitial space was calculated considering the in vitro macrophage uptake using primary cells, as previously published by some of the authors [21]. Subsequently, it was scaled up to macrophage clearance in the liver, spleen, and lungs as described in the Eqs. 3 and 4:
Clint,MPS,organ = \({Cl}_{int,MPS}\times {N}_{macrophages}\times {W}_{organ}\), Eq. 3
ClMPS,organ = \(\frac{(Clint,macrophage,organ \times Qorgan)}{(Clint,macrophage,organ + Qorgan)}\) , Eq. 4
where Clint,MPS, Nmacrophages and Worgan represent the in vitro macrophage intrinsic clearance (mL/min/cell), the number of macrophages per gram of organ and the weight of the organ in grams (liver, spleen, and lungs). The units of Clint,MPS,organ and ClMPS,organ were mL/h and L/h for rat and human models, respectively.
Metabolism and elimination
It was assumed in the model that the NP is phagocytosed by macrophages, dissolved, and doxorubicin released according to a macrophage release constant rate (MPSrelease, h− 1). Doxorubicin is subsequently metabolized and eliminated by apparent systemic clearance. A release rate constant (kdeg, h− 1) based on previous publication [22] was applied to consider the release of doxorubicin from the liposome in all compartments as shown in Fig. 1.
Model parametrization
All the NP and drug-specific parameters for the model are described in Table 2. Since the in vitro parameters MPSrelease and Clint,MPS used as input in the model were not specific to Doxil®, initially they were adjusted in the rat model to match tissue accumulation values observed in vivo. After validation, these values were used in the human model to predict NP accumulation in different organs, since detailed human tissue distribution data is missing. The Nmacrophages per gram of tissue in rats was assumed the same as in mice. The values used in the model were: 8.6 × 105 for human liver, 1 × 107 for rat liver, 2 × 106 for human spleen, 4.5 × 106 for rat spleen, 1.1 × 106 for human lungs, and 3 × 107 for rat lungs [21]. The blood to plasma ratio (R) of Doxil® was considered 0.55 (Table 2) for rat and human (haematocrit in rats slightly higher than in humans) [20], since Doxil® does not bind to erythrocytes and is presented totally in the plasma. It was considered that a reduced or irrelevant amount of protein is adsorbed to its surface (fup = 1), because it is a pegylated liposomal NP [20].
Table 2
Input parameters for the Doxil® and doxorubicin models in rat and human.
Parameter | Doxil® | Doxorubicin |
Physicochemical | | |
Molecular weight (g/mol) | - | 578 [23] |
Log Po:w | - | 1.27 [23] |
pKa | - | 9.53 (strongest acidic) 8.94 (strongest basic) [23] |
fup | 1 | 0.02 [23] |
R | 0.55 * | 2.4 [24] |
PSA (Å2) | - | 206 [23] |
HBD | - | 6 [23] |
Distribution | | |
Vss correction factor | Rat | - | 20 |
Human | - | 5 |
kdeg (h− 1) | 0.025 [22] | - |
MPSrelease (h− 1) | 9 x 10− 4 [21] 9 x 10− 3 * | - |
Clint,MPS (mL/h/cell) | 2.16 x 10− 7 [21] 2.16 x 10− 8 * | - |
Elimination | | |
Clsyst | Rat (mL/h) | - | 321 ± 43.8 [24] |
Human (L/h) | - | 25.3 (13.3–35.2) [25] |
Log Po:w partition coefficient between octanol and water, pKa acid dissociation constant, fup fraction of drug unbound in plasma, R blood to plasma drug ratio, PSA polar surface area, HBD number of hydrogen bond donors, Vss volume of distribution at steady-state, kdeg release constant rate, MPS release macrophage release constant rate, Clint macrophage intrinsic clearance in primary macrophage cells, Clsyst apparent systemic clearance, - not applicable, *adjusted in the model considering haematocrit of 0.45. |
Model qualification
The m-PBPK model was qualified using the administration of 6 mg/kg and 50 mg/m2 of doxorubicin intravenously (IV) in bolus to rats and humans, respectively [26, 27]. The model was assumed to be qualified if the AAFE was below 2 when compared the predicted pharmacokinetic (PK) parameters AUC0 − inf, t1/2, Vd and Cl to the observed data. The model was initially qualified with the IV administration of the released drug alone was to characterize the elimination phase (Clsyst) to be subsequently applied to the NP model. The administration of 1 mg/kg of Doxil® IV in bolus to rats was simulated (Fig. 2) and the NP model was qualified against observed data [28, 29] in rats to adjust the parameters MPSrelease and Clint,MPS [21]. Finally, the same parameters used in the rat model were applied to the human NP m-PBPK model and the administration of 50 mg/m2 of Doxil® IV (time of infusion 30 minutes) to humans was simulated (Fig. 3) and PK parameters were compared to the observed data [27].
Prediction of tissue distribution in rats and humans
Predictions of tissue accumulation in both rat and human models were simulated with administrations of 1 mg/kg and 50 mg/m2 of Doxil® IV, respectively. The total concentration in tissue after 24 hours was calculated based on encapsulated + unencapsulated doxorubicin concentration. A sensitivity analysis of the rat model was performed considering the changes of Doxil® maximum concentration at 24 h in the liver, spleen, and lungs to a range variation of ± 10 and ± 5 times the initial value parameters: kdeg, MPSrelease, Clint,MPS.