Part 2: subjects will be randomized in a 1:1 ratio between the 2 dose levels recommended by the SRC to better understand the exposure/efficacy/toxicity relationship. At the end of Part 2, the SRC will evaluate the safety, tolerability, preliminary efficacy, and PK data to determine the Recommended Phase 3 dose (RP3D) of HMPL-523. primaryPurpose: TREATMENT masking: NONE count: 48 type: ESTIMATED name: HMPL-523 measure: Safety and tolerability of HMPL-523 in adult subjects with primary ITP measure: Dose Limiting Toxicities measure: Cmax (maximum plasma drug concentration) measure: AUCtau (area under the concentration-time curve over a dosage interval) measure: Tmax (time to reach maximum plasma drug concentration) measure: Cmin (minimum plasma drug concentration) sex: ALL minimumAge: 18 Years stdAges: ADULT stdAges: OLDER_ADULT facility: Center for Cancer and Blood Disorders status: RECRUITING city: Bethesda state: Maryland zip: 20817 country: United States name: Ralph V Boccia, MD role: CONTACT name: Ralph V Boccia, MD role: PRINCIPAL_INVESTIGATOR lat: 38.98067 lon: -77.10026 facility: Massachusetts General Hospital status: NOT_YET_RECRUITING city: Boston state: Massachusetts zip: 02114 country: United States name: David J Kuter, MD role: CONTACT name: David J Kuter, MD role: PRINCIPAL_INVESTIGATOR lat: 42.35843 lon: -71.05977 facility: Taussig Cancer Institute status: NOT_YET_RECRUITING city: Cleveland state: Ohio zip: 44106 country: United States name: Alan Lichtin, MD role: CONTACT name: Alan Lichtin, MD role: PRINCIPAL_INVESTIGATOR lat: 41.4995 lon: -81.69541 facility: Peninsula Private Hospital status: NOT_YET_RECRUITING city: Frankston state: Victoria country: Australia name: Huy Tran, MD role: CONTACT name: Huy Tran, MD role: PRINCIPAL_INVESTIGATOR lat: -38.14458 lon: 145.12291 facility: The Perth Blood Institute (PBI) Hollywood Specialist Centre status: NOT_YET_RECRUITING city: West Perth state: Western Australia country: Australia name: Ross Baker, MD role: CONTACT name: Ross Baker, MD role: PRINCIPAL_INVESTIGATOR lat: -31.94896 lon: 115.84199 facility: Marien Hospital Dusseldorf status: NOT_YET_RECRUITING city: Düsseldorf zip: 40479 country: Germany name: Stefanie Groepper, MD role: CONTACT name: Stefanie Groepper, MD role: PRINCIPAL_INVESTIGATOR lat: 51.22172 lon: 6.77616 facility: Sykehuset Ostfold Kalnes (fosta) / Osfold Hospital Trust (MSL) status: NOT_YET_RECRUITING city: Grålum zip: 1714 country: Norway name: Waleed Ghanima, MD role: CONTACT name: Waleed Ghanima, MD role: PRINCIPAL_INVESTIGATOR lat: 59.29048 lon: 11.06668 facility: Hospital del Mar Barcelona status: NOT_YET_RECRUITING city: Barcelona zip: 08003 country: Spain name: Blanca Gonzalez, MD role: CONTACT name: Blanca Gonzalez, MD role: PRINCIPAL_INVESTIGATOR lat: 41.38879 lon: 2.15899 facility: Hospital Universitari Vall d'Hebron status: NOT_YET_RECRUITING city: Barcelona country: Spain name: David Ferreiras, MD role: CONTACT name: David Ferreiras, MD role: PRINCIPAL_INVESTIGATOR lat: 41.38879 lon: 2.15899 facility: University de Burgos status: NOT_YET_RECRUITING city: Burgos zip: 09001 country: Spain name: Jose Gonzalez Lopez, MD role: CONTACT name: Jose Gonzalez Lopez, MD role: PRINCIPAL_INVESTIGATOR lat: 42.34106 lon: -3.70184 facility: Hospital Gregorio Maranon Madrid status: NOT_YET_RECRUITING city: Madrid zip: 28007 country: Spain name: Cristina Izquierdo, MD role: CONTACT name: Cristina Izquierdo, MD role: PRINCIPAL_INVESTIGATOR lat: 40.4165 lon: -3.70256 facility: Clinica Universidad de Navarra status: NOT_YET_RECRUITING city: Madrid zip: 28027 country: Spain name: Carlos Garcia Grande, MD role: CONTACT name: Carlos Garcia Grande, MD role: PRINCIPAL_INVESTIGATOR lat: 40.4165 lon: -3.70256 facility: Hospital Infanta Leonor status: NOT_YET_RECRUITING city: Madrid zip: 28031 country: Spain name: Angeles Foncillas, MD role: CONTACT name: Angeles Foncillas, MD role: PRINCIPAL_INVESTIGATOR lat: 40.4165 lon: -3.70256 facility: Fundacion Jimenez Diaz status: NOT_YET_RECRUITING city: Madrid zip: 28040 country: Spain name: Pilar Llamas Sillero, MD role: CONTACT name: Pilar Llamas Sillero, MD role: PRINCIPAL_INVESTIGATOR lat: 40.4165 lon: -3.70256 facility: Hospital Morales Meseguer status: NOT_YET_RECRUITING city: Murcia zip: 30008 country: Spain name: Maria Lozano, MD role: CONTACT name: Maria Lozano, MD role: PRINCIPAL_INVESTIGATOR lat: 37.98704 lon: -1.13004 hasResults: False