A diagram showing the balance between antibody penetration and systemic or local clearance. A) A sequence of four time points illustrates the penetration of antibody into the tumor while free antibody remains in the blood. Once the plasma concentration has cleared, no more free antibody exists to diffuse deeper into the tumor. A clearance modulus greater than unity indicates systemic clearance occurs before all cells out to the target distance R are targeted. B) Catabolism of antibody counteracts penetration into tumor tissue. In this illustration, the steady-state concentration of plasma antibody is only sufficient to target one cell layer away from the capillary. The green shaded area indicates the region saturated with antibody, while several individual antibodies are shown to explicitly depict catabolism. The Thiele modulus value greater than one indicates that the antibody is catabolized before reaching all of the cells out to the target distance R.

Examples of antibody heterogeneity in tumors. A) In the left panel, a tissue slice is incubated with antibody in vitro, demonstrating fairly uniform antigen distribution. However in the right panel, in vivo injection of antibody results in heterogeneous distribution. 0.2 μg of an anti-HLA IgG were injected, and a sample of an RCC xenograft was taken at 24 h [161]. B) As part of an extensive body of modeling and experimental work on what they termed the “binding site barrier”, Weinstein and coworkers demonstrated this spatial heterogeneity at a low dose (30 μg) of a radiolabeled IgG (D3), in an intradermal L10 tumor in guinea pigs, sampled at 6 h [93]. Adjacent tissue slices were stained for total antigen (left) and antibody (right). C) In an influential demonstration of heterogeneity for high affinity antibody fragments, Adams et al. showed that a picomolar affinity scFv (red) penetrated only several cell layers away from capillaries (yellow). 100 micrograms of anti-Her2 scFv was injected to nephrectomized mice with SK-OV-3 xenografts, and sampled at 24 h [70]. D) In this case, heterogeneity is shown for an actual therapeutic molecule (10 mg/kg Trastuzumab, shown in green) in a mouse xenograft (F2-1282 tumor) 24 h after injection1[16]. The drug moves only several cell layers away from vasculature (shown in red), leaving many unreached cells (counterstained blue). E) The movement of a sharp antibody front in a spheroid of tumor cells is here demonstrated [162]. A 200 μm diameter spheroid of LNCaP-LN3 cells was incubated with 10 μg/mL J591-FITC IgG for 1, 2,3, and 24 24 h. Imaged by confocal microscopy, the antibody is seen to penetrate as a sharp front.

A schematic illustration of the key rate processes in antibody distribution in both vascularized tumors and micrometastases. Although many of the processes are the same, and the length scales are similar as well (R∼100 μm for each), there is a dramatic difference in the extravasation flux across the capillary walls, which is much smaller in bulk tumors due to the buildup of hydrostatic pressure [60]. Consequently the antibody concentration at the boundary of tumor tissue is considerably lower in vascularized bulk tumors, leading to a smaller concentration gradient and lower diffusive flux through the tumor than in the micrometastatic case.