Introduction
Liposome formulations of vincristine have been in clinical development for over a decade [
5,
14,
46,
48]. Unlike anthracyclines,
vinca alkaloids are notoriously more difficult to formulate stably in liposomes [
10,
44,
48,
51]. Simple pH- or ammonium sulfate gradients are sufficient to load and stabilize doxorubicin in liposomes [
1,
7,
15,
30], helping to give rise to long circulating and highly active formulations that include pegylated liposomal doxorubicin (Doxil
®, Alza/Johnson&Johnson; Palo Alto, CA). Schedule-dependent drugs such as vincristine [
17,
18] would be expected to benefit to an even greater extent from a stable liposome formulation, with controlled release of the drug from the liposomal carrier, thus resulting in an extended duration of exposure of the target cancer tissue to the active drug [
2,
12].
In addition, although liposomal vincristine has been most extensively studied in leukemias and lymphomas [
31,
32,
43,
44,
46], vincristine, as well as vinblastine, have also shown activity in the treatment of certain solid tumors, including neuroblastomas [
38,
39] and melanoma [
24,
25]. Accumulation of nanoparticle drug carriers, such as liposomes, in solid tumors is generally governed by a relatively selective, but slow extravasation from a “leaky” tumor microvasculature, as a part of what is commonly referred to as the enhanced permeability and retention (EPR) effect [
9,
28]. To fully take advantage of the EPR phenomenon in treating solid tumors, the liposomal formulation must be engineered to retain its active contents for the time needed to effectuate extravasation, or approximately 24–48 h [
9,
13,
21], and thus allow for release of the active drug primarily in the near vicinity of the tumor cells.
Finally, newer generations of liposomal delivery technologies include active targeting moieties such as antibodies to direct the liposomal drugs specifically to receptor-overexpressing tumor cells [
36,
42]. Because the targeting ligand is not directly conjugated to the drug itself, but instead indirectly to the carrier, stable encapsulation is an absolute requirement to ensure that the drug arrives intact at the target site and reduces exposure to non-target tissues that will arise if the drug becomes bioavailable prematurely while in the circulation [
10,
36,
42].
A variety of strategies has been employed to improve the stability of liposomal vincristine formulations. The modification of the lipid composition, and substitution of sphingomyelin for phosphatidylcholine in the formulation, substantially improved the stability of encapsulation for cholesterol-containing formulations [
48]. The introduction of fully saturated dihydrosphingomyelin into the formulation has further improved its stability [
19]. The use of high drug-to-lipid ratio formulations to increase intraliposomal concentrations of vincristine, and thus reduce its solubility, has also been shown to improve stability [
20]. Finally, the use of dextran sulfate to complex vincristine has been used to limit its diffusion from a liposomal carrier, albeit at the expense of decreased antitumor activity [
52].
Here, we describe the preparation of novel liposomal vincristine and vinblastine formulations stabilized intraliposomally with the sulfated non-polymeric polyol sucrose octasulfate. These preparations were highly stable in vivo despite the absence of sphingomyelin in the formulation, and even at relatively low vincristine to phospholipid ratios. Immunotargeted versions were prepared through conjugation of a human anti-HER2 scFv to the surface of the carrier, and shown to result in target-specific cytotoxicity in breast cancer cells in culture and improved antitumor efficacy in human breast tumor xenografts in vivo. This proof-of-concept study suggests that immunotargeting of liposomal vincristine to solid tumors is feasible when the nanocarriers are sufficiently stabilized to limit drug leakage in the circulation.
Discussion
Vinca alkaloids such as vincristine, vinblastine, and vinorelbine are widely used cytotoxic drugs that elicit their effects through disruption of microtubules, resulting in metaphase arrest in dividing cells [
41]. Due to their mechanism of action,
vinca alkaloids are schedule dependent drugs and thus their activity is affected by the duration of exposure to the drug [
17,
18]. As such,
vinca alkaloids would benefit from a controlled release dosage form that would effectively prolong the duration of exposure over extended periods of time. Liposomal nanocarriers represent one such dosage form that has been extensively studied for its ability to prolong the pharmacokinetics and subsequent exposure of various drugs, including
vinca alkaloids [
2,
9,
12].
However, liposome formulations of
vinca alkaloids are considerably more difficult to stabilize in vivo when compared to the more widely studied anthracyclines. Possibly, due to the high propensity of doxorubicin molecules for self-association, doxorubicin forms highly stable precipitates inside liposomes following loading using simple pH or ammonium sulfate-gradients [
22], and demonstrates release rates on the order of 100 h in vivo [
44]. Using similar lipid compositions and gradient-loading strategies allowed achieving the T
½ of vincristine leakage from the liposome equal to 17 h [
48]. The rapid release from the nanoparticle carrier is unfavorable for the drug’s ability to benefit from the EPR effect in treating solid tumors. Although extravasation efficiency varies depending on tumor location [
16] and the various physicochemical properties of the liposomal carrier, including size and surface charge, the maximum accumulation of long-circulating liposomes in tumors has been reported as generally occurring at about 24–48 h post administration [
9,
13,
21]. In humans, the circulation lifetimes can even be substantially longer than observed in animal models [
27,
49]. Thus, if a substantial proportion of the encapsulated drug is released prior to the liposomes reaching the tumor; the drug is deprived of an advantage of the depot effect whereby the drug is released locally in close proximity to the tumor cells.
The requirement for stable encapsulation is even more important for ligand-targeted formulations, where specific delivery to receptor-overexpressing tumor cells is not possible if the drug is released prematurely before reaching its site of action [
36,
42]. The fact that the ligand is not covalently conjugated to the active therapeutic agent can be a considerable advantage over other drug immunoconjugates or immunotoxins that require complicated linker strategies, and where species-dependent differences in linker hydrolysis rates complicate the development of these agents [
50]. Controlling the rates of drug release through the nanocarrier’s physicochemical properties and drug encapsulation technology [
12], obviates the need for hydrolysis of the chemical linkers. However, perhaps due in part to the relative instability of vincristine liposome formulations, immunotargeted formulations of vincristine have been primarily studied in readily vascularly accessible hematological cancers [
43,
44].
A variety of approaches has been advanced to improve in vivo formulation stability, with varying degrees of success depending on the specific
vinca alkaloid being delivered. Bally, Mayer, and coworkers successfully substituted sphingomyelin for phosphatidylcholine in cholesterol-containing liposomal vincristine formulations to limit the diffusion of the drug across the membrane, nearly doubling the half-life of vincristine release from 17.1 to 33.3 h [
48]. These liposomes, termed “Sphingosomes”, are currently being developed by Hana Biosciences (South San Francisco, CA) for the treatment of non-Hodgkin’s lymphoma and acute lymphoblastic leukemia, and have been studied in both Phase I and II clinical trials [
4,
14,
46]. A modification of this approach uses fully hydrogenated sphingomyelin to further stabilize the formulation against in vivo drug leakage, resulting in an increase in circulation lifetime of the sphingosomal vincristine in mice to 13.2 h compared to 9.4 h for the control egg sphingomyelin/cholesterol formulation at the injected lipid dose of 150 μmol PL/kg [
19].
High drug-to-lipid ratios can also enhance the formation of intraliposomal drug precipitate, and thus reduce the intraliposome pool of the dissolved, membrane-permeable form of the drug available for transmembrane diffusion, which, along with the permeability constant, determines the drug release rate from the liposomes once in the circulation [
12,
20]. The combination of sphingomyelin/cholesterol formulations together with high drug-to-lipid ratios perhaps exceeding the drug solubility product within the liposome can finally give rise to liposomal vincristine with impressive stability (
T
1/2 for release of 65 h) [
20], however, at the expense of being limited to high drug-to-lipid ratios resulting in corresponding reductions in the lipid dose being administered for a given dose of drug. This may lead to potentially undesirable consequences for the liposome blood clearance, as conventional (non-PEGylated) liposome formulations display dose-dependent pharmacokinetics and thus lower lipid doses result in more rapid clearance via the mononuclear phagocyte system [
2,
9,
12]. Although PEGylated liposomes display pharmacokinetics that are less dependent on the administered lipid dose, the further reduction of administered lipid doses of a liposomal carrier necessitated by a combination of a high potency drug with a high drug-to-phospholipid ratio may cause increased clearance of even PEGylated liposomal drugs [
12,
23].
While the above-discussed release modification methods [
20] did result in slower release rates for vincristine, the results for vinorelbine (
T
1/2 = 11.0 h) or vinblastine (
T
1/2 = 14.7 h), have not been as successful [
45,
51]. This correlates with the increased hydrophobicity of vinblastine and vinorelbine, relative to vincristine [
26], and thus their greater membrane permeability. Our approach has been to develop a delivery strategy that can effectively stabilize both the hydrophilic and hydrophobic
vinca alkaloids, as well as achieve this stability at low-to-moderate drug-to-lipid ratios where circulation lifetimes will not be compromised. We have previously discovered that a loading strategy that employs a di- or tri-alkylammonium salt of highly sulfated, non-polymeric polyol, sucrose octasulfate, to load and stabilize weakly basic amphipathic drugs intraliposomally resulted in surprising improvements in circulation lifetimes and in vivo drug retention by the carrier [
8,
11]. We hypothesize that very high charge density in combination with the multivalent ionic character and compactness of the molecule unachievable with the previously employed polymeric polyanions [
52] makes sucrose octasulfate a better agent to immobilize a cationic drug, such as a vinca alkaloid, inside the liposome, while the use of exchangeable substituted ammonium cation with the ionic radius larger than ammonium itself helps to reduce the amount of exchangeable cation immobilized by the non-exchangeable highly charged polyvalent anion and therefore improves the completeness of the drug-for-cation exchange across the liposome membrane in the course of the drug loading. Here, we have applied this technology (which is refered to below as “nanoliposomal formulations” or “nanoliposomes”) to both vincristine and vinblastine (Fig.
1a, b) to see if these
vinca alkaloids could be stabilized under a range of formulation conditions.
As described above, we were able to stabilize VCR in liposomes, such that the half life of release was 104.5 h for a 101.6 nm liposome loaded at 104 g VCR/mol PL (~100 g VCR/g lipid; Fig.
3c), which is a 6.7-fold improvement relative to the sphingosomal formulation loaded at the same drug-to-lipid ratio [
20], thus demonstrating significant retention of vincristine is possible even in the absence of the highly cohesive lipid compositions that include sphingomyelin and cholesterol. Non-pegylated liposomes typically also display pharmacokinetics that are dependent on size, with smaller sizes being longer circulating [
9,
12]. There is concern with drug-loaded liposomes is that the high radius of membrane curvature in smaller liposomes can sometimes cause membrane defects that give rise to increased rates of drug leakage. However, for the smallest VCR formulation (76.8 nm) having the lowest drug to lipid ratio (95.1 g VCR/mol PL), the small size did not appear to have a detrimental effect on circulation lifetimes or in vivo stability.
Although this manuscript focuses on VCR, a single formulation of VBL was also studied to demonstrate that the methodology was not unique to a single
vinca alkaloid. The nanoliposomal formulation of vinblastine was also markedly more stable (
T
1/2 = 41.3 h at drug-to-lipid ratio of 0.14 g VBL/g lipid) than the previously described sphingosomal formulations at either the high or low drug-to-lipid ratio studied (
T
1/2 = 3.1 h for 0.1 g VBL/g lipid and 14.7 h for the 0.3 g VBL/g lipid) [
51]. Importantly, for both drugs the stability was high enough to allow liposomes sufficient time to accumulate in solid tumors and thus to take full advantage of the EPR phenomenon and molecular targeting of solid tumors. Additionally, extended circulation lifetimes were maintained over a range of formulation parameters, including at comparatively small sizes (76.8 nm; Table
1), thus suggesting that modifications that may allow for increased extravasation and accumulation in the tumor may also be permissible and desirable for targeted formulations.
Immunoliposomes were prepared through conjugation of an anti-HER2 scFv (F5) to the surface of the liposome via a maleimide-activated PEG-DSPE anchor [
33,
34]. We have previously demonstrated that antibody fragments capable of inducing internalization upon binding to the tyrosine kinase receptors, EGFR and HER2/neu, were able to improve the antitumor activity of liposomally encapsulated anticancer drugs [
29,
35,
37]. HER2-specific targeting of nanoliposomal VCR in vitro not only restored the liposomal drug efficiency to the level of the free drug, but also, surprisingly, surpassed it, giving evidence that despite effective stabilization against drug leakage, intraliposomal sucroseoctasulfate afforded sufficient bioavailability of the encapsulated VCR. Targeted antitumor efficacy for F5-immunoliposomal vincristine was demonstrated also in vivo in a HER2-overexpressing breast cancer model (Fig.
5). Efficacy was significantly improved for the HER2-targeted nanoliposomal vincristine when compared to nontargeted nanoliposomal vincristine (
p = 0.025) demonstrating the sufficient stabilization of encapsulation can result in targeted antitumor activity in solid tumors. Vincristine is used clinically in the treatment of acute leukemia, non-Hodgkin’s malignant lymphomas, Hodgkin’s disease, neuroblastomas, rhabdosarcomas, and Wilm’s tumors [
41]. It has also been clinically in the treatment of melanomas [
24,
25] and small cell lung cancer [
6]. Vincristine is not used widely in the treatment of breast cancer, although vinorelbine is used in third line treatment. However, changes in the pharmacokinetics, bioavailability, and tumor exposure of the drug resulting from stable liposome encapsulation, as well as the ability to be molecularly targeted may alter the range of cancers susceptible to treatment with vincristine, which has a relatively nonspecific mechanism of action. Alternatively, this proof-of-concept study suggests that for the first time vincristine may be targeted for the treatment of solid tumors, and that similar formulations using different antibodies targeted to melanoma, neuroblastomas, or small cell lung cancer now have the potential for being efficacious in treating these cancers.