Innovations and Patent Trends in the Development of USFDA Approved Protein Kinase Inhibitors in the Last Two Decades

Protein kinase inhibitors (PKIs) are important therapeutic agents. As of 31 May 2021, the United States Food and Drug Administration (USFDA) has approved 70 PKIs. Most of the PKIs are employed to treat cancer and inflammatory diseases. Imatinib was the first PKI approved by USFDA in 2001. This review summarizes the compound patents and the essential polymorph patents of the PKIs approved by the USFDA from 2001 to 31 May 2021. The dates on the generic drug availability of the PKIs in the USA market have also been forecasted. It is expected that 19 and 48 PKIs will be genericized by 2025 and 2030, respectively, due to their compound patent expiry. This may reduce the financial toxicity associated with the existing PKIs. There are nearly 535 reported PKs. However, the USFDA approved PKIs target only about 10–15% of the total said PKs. As a result, there are still a large number of unexplored PKs. As the field advances during the next 20 years, one can anticipate that PKIs with many scaffolds, chemotypes, and pharmacophores will be developed.


Introduction
Protein kinases (PKs) are ubiquitous intracellular and cell surface enzymatic proteins that selectively catalyzes phosphate group's relocation from ATP, GTP, and other phosphate donors to protein substrates [1]. The PKs mainly catalyze the relocation of a γ-phosphatase group of ATP to the oxygen atom of the -OH group of threonine, serine, and tyrosine residues in peptides/polypeptides, thereby making a conformational variation from an inactive to an active form [1,2]. They constitute an extensive family of structurally related enzymes that are known to be implicated in almost all the signal transduction activities, frequently with cascades of phosphorylation proceedings taking place within the cell [3]. The signal transduction involves the reversible phosphorylation of proteins that helps to regulate mature proteins by altering their structure and function [4,5]. To date, nearly 535 human PKs have been identified [6], wherein more than 478 belong to a superfamily whose catalytic domains are sequentially interrelated. These PKs are additionally categorized into groups, families, and subfamilies established on their biochemical activities. The main two classifications are Serine/threonine PKs and Tyrosine-specific PKs [5]. The seven significant groups with the description of families, subfamilies, and functions are listed in Table 1.
TKs form a distinct group, which phosphorylates proteins on tyrosine, whereas others phosphorylate serine and threonine residues. In addition to this category, there are atypical kinases, which are not related to any sequence resemblance to characteristic kinases but are well recognized for their enzymatic activity similar to specific kinases. Some kinases are believed to lack the catalytic domain for effective phosphorylation and are called pseudokinases. Still, they are distributed across all kinase families, indicating that an absence of catalysis is not a formal barricade to the evolution of unique or irreplaceable biological functions [7].

USFDA Approved Protein Kinase Inhibitors
In 2001, the USFDA approved the marketing of the first clinical PKI, imatinib. Since then, the USFDA has approved about 70 PKIs for clinical use (Table 2) ( Figure 1). The data provided in Table 2 have been obtained from USFDA's Orange Book website (https: //www.accessdata.fda.gov/scripts/cder/ob/index.cfm?resetfields=1 (accessed on 31 May 2021) using the drug's name.

Patent Searching
The patent searching was performed using the Sci-finder database (CAS Number search, and the exact structure search of each TKI), USFDA's Orange Book website (mentioned above), and the Drugbank's website (https://go.drugbank.com/ (accessed on 31 May 2021)) using the drug's name. The patents disclosing the specific TKI, its marketed active pharmaceutical ingredient, and important polymorphs from the innovative company for the first time were identified and included in this review. The patents of each TKI that claim its treatment methods, dosage forms, formulations, drug combinations, particle size, impurity, preparation process, intermediates, etc., have been excluded from this review. The expiry dates of the selected patents were calculated (20 years from the

Patent Searching
The patent searching was performed using the Sci-finder database (CAS Number search, and the exact structure search of each TKI), USFDA's Orange Book website (mentioned above), and the Drugbank's website (https://go.drugbank.com/ (accessed on 31 May 2021)) using the drug's name. The patents disclosing the specific TKI, its marketed active pharmaceutical ingredient, and important polymorphs from the innovative com- pany for the first time were identified and included in this review. The patents of each TKI that claim its treatment methods, dosage forms, formulations, drug combinations, particle size, impurity, preparation process, intermediates, etc., have been excluded from this review. The expiry dates of the selected patents were calculated (20 years from the patent application filing date comprising patent term extension, if any). Sometimes, the drug's patent term is extended up to five years based on the USPTO's laws. Accordingly, the expiry dates of the selected patients were also verified from the USPTO's website. It was also observed that some TKIs were disclosed in different patents of the same patent family and had other expiry dates. In such cases, the patent that had a more extended expiry date was selected for this review because the generic launch of the drug is based on the expiry date of the drug's patent. The legal status of the patents cited herein was obtained from the website of USPTO (https://portal.uspto.gov/pair/PublicPair (accessed on 31 May 2021)).

Summary of the Patents
The proprietary name, approved dosage form, approval date, and marketing status of each marketed PKIs are mentioned in Table 2. The patent number, applicant/assignee, expiry date, and legal status of the cited patents of each PKI are provided in Table 3. A brief description of the PKIs and their important patents are provided below.

Axitinib
Axitinib ( Figure 17) is a pyridine based indazolylphenyl thioether (MF: C 22 H 18 N 4 OS; MW: 386.47; CAS Number: 319460-85-0) [66]. US6534524B1 relates to indazole compounds as PTKIs and their pharmaceutical compositions to treat diseases linked with undesirable angiogenesis and cellular proliferation. It claims axitinib specifically [67]. US8791140B2 claims crystalline forms of axitinib that have advantages in bioavailability, stability, manufacture ability, and suitability for bulk preparation [68]. pounds as PTKIs and their pharmaceutical compositions to treat diseases linked with undesirable angiogenesis and cellular proliferation. It claims axitinib specifically [67]. US8791140B2 claims crystalline forms of axitinib that have advantages in bioavailability, stability, manufacture ability, and suitability for bulk preparation [68].

Cabozantinib S-Malate
Cabozantinib S-malate ( Figure 21) [78]. US7579473B2 relates to quinazolines and quinolines as TKIs, and their pharmaceutical compositions to treat psoriasis, multiple sclerosis, and rheumatoid arthritis. It claims cabozantinib and its salts [79]. US8877776B2 claims cabozantinib (L)-malate salt having desirable solubility and chemical/physical stability to develop a tablet/capsule dosage forms for intended use [80].

Nintedanib Esylate
Nintedanib esylate ( Figure 29) [100]. US6762180B1 states indolinone derivatives as PTKIs, synthesis, and compositions to treat proliferative sicknesses. It claims nintedanib and its salts [101]. US7119093B2 claims a stable nintedanib esylate salt specifically characterized by good crystallinity and low amorphization during grinding and compression. This salt is claimed to have good physicochemical characteristics to support quality dosage forms [102].

Lenvatinib Mesylate
Lenvatinib mesylate ( Figure 31) [106]. US7253286B2 reports nitrogen-containing aromatic derivatives and salts or hydrates thereof to treat various diseases associated with abnormal angiogenesis. It claims lenvatinib and its pharmacologically active salts [107]. US7612208B2 claims a crystalline form of lenvatinib mesylate with improved features (physical/pharmacokinetics) compared to the free-form [108].  [106]. US7253286B2 reports nitrogen-containing aromatic derivatives and salts or hydrates thereof to treat various diseases associated with abnormal angiogenesis. It claims lenvatinib and its pharmacologically active salts [107]. US7612208B2 claims a crystalline form of lenvatinib mesylate with improved features (physical/pharmacokinetics) compared to the free-form [108].

Ribociclib Succinate
Ribociclib succinate ( Figure 35) [116]. US8415355B2 discloses pyrrolopyrimidine compounds, the process for their preparation, and their pharmaceutical compositions to treat a disease linked with CDK 4 inhibition. It claims ribociclib and its salts [117]. US9193732B2 claims succinate salt of ribociclib that has good stability, non-hygroscopicity, and good solubility. These features make this salt a suitable salt to develop the desired formulation [118].
It claims acalabrutinib and its salts [133]. US9796721B2 claims a stable and non-hygroscopic anhydrate crystal form of acalabrutinib as having advantageous parameters for making quality pharmaceutical compositions [134].

Fostamatinib Disodium Hexahydrate
Fostamatinib disodium hexahydrate (Figure 47), a phosphate prodrug of tamatinib, is a pyrimidine based pyrido [3,2- [147]. US7449458B2 reports prodrugs of pharmacologically active 2,4-pyrimidinediamine derivatives, intermediates thereof, the process of manufacturing them, and pharmaceutical compositions comprising them to treat diseases mediated by the activation of PTKs. It claims fostamatinib disodium hexahydrate, which has increased solubility concerning the parent phosphate prodrug [148]. US8163902B2 claims a thermodynamically stable crystalline form of fostamatinib disodium hexahydrate that is stable over a wide range of relative humidity and requires substantial heating to lose its water molecules. This property makes it a suitable API to develop the desired dosage form [149].   [145]. US8501758B2 provides pyrazole based pyrimidine and pharmaceutical compositions comprising them to treat disorders associated with the deregulated activity of B-Raf. It claims encorafenib and its salts [146].

Fostamatinib Disodium Hexahydrate
Fostamatinib disodium hexahydrate (Figure 47), a phosphate prodrug of tamatinib, is a pyrimidine based pyrido [3,2- [147]. US7449458B2 reports prodrugs of pharmacologically active 2,4-pyrimidinediamine derivatives, intermediates thereof, the process of manufacturing them, and pharmaceutical compositions comprising them to treat diseases mediated by the activation of PTKs. It claims fostamatinib disodium hexahydrate, which has increased solubility concerning the parent phosphate prodrug [148]. US8163902B2 claims a thermodynamically stable crystalline form of fostamatinib disodium hexahydrate that is stable over a wide range of relative humidity and requires substantial heating to lose its water molecules. This property makes it a suitable API to develop the desired dosage form [149].

Fostamatinib Disodium Hexahydrate
Fostamatinib disodium hexahydrate (Figure 47), a phosphate prodrug of tamatinib, is a pyrimidine based pyrido [3,2- [147]. US7449458B2 reports prodrugs of pharmacologically active 2,4-pyrimidinediamine derivatives, intermediates thereof, the process of manufacturing them, and pharmaceutical compositions comprising them to treat diseases mediated by the activation of PTKs. It claims fostamatinib disodium hexahydrate, which has increased solubility concerning the parent phosphate prodrug [148]. US8163902B2 claims a thermodynamically stable crystalline form of fostamatinib disodium hexahydrate that is stable over a wide range of relative humidity and requires substantial heating to lose its water molecules. This property makes it a suitable API to develop the desired dosage form [149].

Duvelisib Hydrate
Duvelisib hydrate ( Figure 48) is a purine based isoquinolone derivative (MF: C22H17ClN6O.H2O; MW: 434.88; CAS Number: 1201438-56-3) [150]. US8193182B2 provides isoquinolin-1(2H)-one derivatives as modulators of PI3 kinase activity and pharmaceutical compositions comprising them to treat diseases associated with P13 kinase activity. It claims duvelisib and its salts [151]. USRE46621E (Reissue of US8809349B2) claims physically and chemically stable polymorphs of duvelisib, salt, solvate, or hydrate that do not readily decompose or change in chemical makeup or physical state for more than 60 months and are suitable to develop the desired dosage forms of the API [152].

Gilteritinib Fumarate
Gilteritinib fumarate (Figure 49) piperazine-piperidine based pyrazine carboxamide derivative (MF: (C29H44N8O3)2.C4H4O4; MW: 1221.50; CAS Number: 1254053-84-3) [153]. US8969336B2 states diamino heterocyclic carboxamide derivatives as having outstanding inhibitory activity against EML4-ALK fusion proteins for use in cancer therapy. It claims gilteritinib and its salts [154]. The gilteritinib fumarate salt is stable in heat, humidity, and storage conditions.   [150]. US8193182B2 provides isoquinolin-1(2H)-one derivatives as modulators of PI3 kinase activity and pharmaceutical compositions comprising them to treat diseases associated with P13 kinase activity. It claims duvelisib and its salts [151]. USRE46621E (Reissue of US8809349B2) claims physically and chemically stable polymorphs of duvelisib, salt, solvate, or hydrate that do not readily decompose or change in chemical makeup or physical state for more than 60 months and are suitable to develop the desired dosage forms of the API [152].

Duvelisib Hydrate
Duvelisib hydrate ( Figure 48) is a purine based isoquinolone derivative (MF: C22H17ClN6O.H2O; MW: 434.88; CAS Number: 1201438-56-3) [150]. US8193182B2 provides isoquinolin-1(2H)-one derivatives as modulators of PI3 kinase activity and pharmaceutical compositions comprising them to treat diseases associated with P13 kinase activity. It claims duvelisib and its salts [151]. USRE46621E (Reissue of US8809349B2) claims physically and chemically stable polymorphs of duvelisib, salt, solvate, or hydrate that do not readily decompose or change in chemical makeup or physical state for more than 60 months and are suitable to develop the desired dosage forms of the API [152].

Gilteritinib Fumarate
Gilteritinib fumarate (Figure 49 [153]. US8969336B2 states diamino heterocyclic carboxamide derivatives as having outstanding inhibitory activity against EML4-ALK fusion proteins for use in cancer therapy. It claims gilteritinib and its salts [154]. The gilteritinib fumarate salt is stable in heat, humidity, and storage conditions.

Tivozanib Hydrochloride Monohydrate
Tivozanib hydrochloride monohydrate ( Figure 70)  for treating tumors, chronic rheumatism, psoriasis, and Kaposi's sarcoma. These patents claim tivozanib and its salts [206,207]. US7166722B2 claims a physically stable crystalline form of tivozanib hydrochloride monohydrate stable under high temperature and humidity. This form is suitable for developing quality dosage forms [208].

Expert Opinion
In 2001, USFDA approved the marketing of the first clinical PKI, imatinib. From 2001 to 31 May 2021, about 70 PKIs have been approved by the USFDA ( Table 2). The USFDA has also approved antibodies as PKIs such as trastuzumab and bevacizumab. A few anti-

Expert Opinion
In 2001, USFDA approved the marketing of the first clinical PKI, imatinib. From 2001 to 31 May 2021, about 70 PKIs have been approved by the USFDA ( Table 2). The USFDA has also approved antibodies as PKIs such as trastuzumab and bevacizumab. A few anti-

Expert Opinion
In 2001, USFDA approved the marketing of the first clinical PKI, imatinib. From 2001 to 31 May 2021, about 70 PKIs have been approved by the USFDA ( Table 2). The USFDA has also approved antibodies as PKIs such as trastuzumab and bevacizumab. A few antibodies are also in the clinical trial (amivantamab and patritumab). This review is limited to small molecules as PKIs. Accordingly, USFDA approved antibodies such as PKIs have not been discussed here. The physicochemical properties of about 55 USFDA approved PKIs from 2001 to 2020 have been described in the literature [22,23]. However, these reports are silent about the patent data of the PKIs reported therein.
The PKIs contain one or more heterocyclic moieties in their structure that can explain the difference in their binding to the target and thus the spectrum of activity. The primary heterocyclic moieties include quinazoline, quinoline, isoquinoline, pyridine, pyrimidine, pyrazole, benzimidazole, indazole, imidazole, indole, carbazole, or their fused structures. This observation suggests that many clinical PKIs have been developed by the chemical modification of a formerly approved drug, and PKs are promiscuous targets. Further, most of the PKIs are marketed as acid-addition salts (hydrochloride, mesylate, tosylate, phosphate, malate, citrate, esylate, fumarate, succinate, and sulfate). This observation indicates the basic nature of the chemical nucleus of the PKIs.
The majority of the PKIs are approved to treat cancer and inflammatory disorders. Some of the PKIs have shown efficacy towards autoimmune diseases, Alzheimer's disease (neflamapimod, tideglusib, and saracitinib), and Parkinson's disease (DNL201). It is also expected that PKIs of PKC/WNK that control the activity of ion transporters may be developed to treat hypertension [214].
The malignant cells have genomic instability, which may cause the development of resistance to PKIs. This phenomenon is the reason for developing 2nd, 3rd, and later generations of PKIs targeting the equivalent PKs and their related disorders [212]. To combat resistance development, scientists are exploring different chemical templates and pharmacophores to develop novel PKIs [22]. Besides, inflammatory conditions do not exhibit genomic instability. Therefore, the PKIs, which are approved to treat inflammatory disorders, seldom demonstrate the development of resistance [22,23].
The main marketed dosage form of about 66 USFDA approved PKIs is either a tablet or capsule (Table 2). These are solid dosage forms. The quality of the formulation of a solid dosage form depends upon the solid-state properties (stability, solubility, compressibility, etc.) of the drug [215]. Therefore, many patents related to salts and polymorphs (mostly crystalline forms) of the USFDA approved PKIs have been obtained by the innovator companies. The innovator companies have done this to capture the market for a longer time.
The development of the PKIs is considered a medical breakthrough. However, the prices of these therapeutics cause financial toxicity. The financial burden can make the patients non-compliant with the treatment instructions as they may take lower doses than the prescribed doses. This causes failure of the treatment [216,217]. One way to avoid financial toxicity is to develop the generic version of a drug [218]. Currently, seven PKIs have been genericized (imatinib, erlotinib, sorafenib, dasatinib, lapatinib, temsirolimus, and everolimus) ( Table 3). These generic versions must have lower prices than the innovator products. The data given in Table 3 also suggest that twelve more PKIs (gefitinib, sunitinib, pazopanib, vandetanib, axitinib, bosutinib, tofacitinib, idelalisib, nintedanib, lenvatinib, midostaurin, and neratinib) may be genericized by 2025 due to basic/compound/governing patent expiry or expiry of the drug exclusivity. It means by the end of 2025, 19 PKIs will have their generic version in the USA market. Besides, it is also expected that the generic version of about 48 PKIs will be available in the USA market by the end of 2030. Thus, it is hoped that the generic availability of these PKIs will reduce the financial toxicity on a patient.
Although great strides have been made in developing small molecule such as PKIs during the past 20 years, this field is still in its infancy. PKs are ubiquitous, and hence specificity has always been an issue regarding the design of new therapies targeting them. The major disadvantage of the existing PKIs is that they target a minor portion of the kinome, with countless clinically significant kinases missing validated inhibitors [22,23]. There are essential kinases without any inhibitors, and this is a critical area for further research. As the field advances during the next 20 years, one can anticipate that PKIs with many scaffolds, chemotypes, and pharmacophores will be developed. Other innovative strategies are also expected soon. A summary of the PKIs is provided in Figure 72.
In conclusion, there is a huge scope for discovering PKIs, and it will dominate other cancer discovery strategies for decades. The rate of discovery of better and selective PKIs having less propensity for resistance development will be faster than the last two decades because of the better understanding of the molecular and structural aspects of the human kinases. The development of PKIs to treat hypertension, Alzheimer's disease, and Parkinson's disease are foreseeable. In conclusion, there is a huge scope for discovering PKIs, and it will dominate other cancer discovery strategies for decades. The rate of discovery of better and selective PKIs having less propensity for resistance development will be faster than the last two decades because of the better understanding of the molecular and structural aspects of the human kinases. The development of PKIs to treat hypertension, Alzheimer's disease, and Parkinson's disease are foreseeable.